Patchwork [17/23] Multi-target support

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Submitter Pedro Alves
Date Sept. 6, 2019, 11:28 p.m.
Message ID <20190906232807.6191-18-palves@redhat.com>
Download mbox | patch
Permalink /patch/34431/
State New
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Comments

Pedro Alves - Sept. 6, 2019, 11:28 p.m.
This commit adds multi-target support to GDB.  What this means is that
with this commit, GDB can now be connected to different targets at the
same time.  E.g., you can debug a live native process and a core dump
at the same time, connect to multiple gdbservers, etc.

Actually, the word "target" is overloaded in gdb.  We already have a
target stack, with pushes several target_ops instances on top of one
another.  We also have "info target" already, which means something
completely different to what this patch does.

So from here on, I'll be using the "target connections" term, to mean
an open process_stratum target, pushed on a target stack.  This patch
makes gdb have multiple target stacks, and multiple process_stratum
targets open simultaneously.  The user-visible changes / commands will
also use this terminology, but of course it's all open to debate.

User-interface-wise, not that much changes.  The main difference is
that each inferior may have its own target connection.

A target connection (e.g., a target extended-remote connection) may
support debugging multiple processes, just as before.

Say you're debugging against gdbserver in extended-remote mode, and
you do "add-inferior" to prepare to spawn a new process, like:

 (gdb) target extended-remote :9999
 ...
 (gdb) start
 ...
 (gdb) add-inferior
 Added inferior 2
 (gdb) inferior 2
 [Switching to inferior 2 [<null>] (<noexec>)]
 (gdb) file a.out
 ...
 (gdb) start
 ...

At this point, you have two inferiors connected to the same gdbserver.

With this commit, GDB will maintain a target stack per inferior,
instead of a global target stack.

To preserve the behavior above, by default, "add-inferior" makes the
new inferior inherit a copy of the target stack of the current
inferior.  Same across a fork - the child inherits a copy of the
target stack of the parent.  While the target stacks are copied, the
targets themselves are not.  Instead, target_ops is made a
refcounted_object, which means that target_ops instances are
refcounted, which each inferior counting for a reference.

What if you want to create an inferior and connect it to some _other_
target?  For that, this commit introduces a new "add-inferior
-no-connection" option that makes the new inferior not share the
current inferior's target.  So you could do:

 (gdb) target extended-remote :9999
 Remote debugging using :9999
 ...
 (gdb) add-inferior -no-connection
 [New inferior 2]
 Added inferior 2
 (gdb) inferior 2
 [Switching to inferior 2 [<null>] (<noexec>)]
 (gdb) info inferiors
   Num  Description       Executable
   1    process 18401     target:/home/pedro/tmp/main
 * 2    <null>
 (gdb) tar extended-remote :10000
 Remote debugging using :10000
 ...
 (gdb) info inferiors
   Num  Description       Executable
   1    process 18401     target:/home/pedro/tmp/main
 * 2    process 18450     target:/home/pedro/tmp/main
 (gdb)

A following patch will extended "info inferiors" to include a column
indicating which connection an inferior is bound to, along with a
couple other UI tweaks.

Other than that, debugging is the same as before.  Users interact with
inferiors and threads as before.  The only difference is that
inferiors may be bound to processes running in different machines.

That's pretty much all there is to it in terms of noticeable UI
changes.

On to implementation.

Since we can be connected to different systems at the same time, a
ptid_t is no longer a unique identifier.  Instead a thread can be
identified by a pair of ptid_t and 'process_stratum_target *', the
later being the instance of the process_stratum target that owns the
process/thread.  Note that process_stratum_target inherits from
target_ops, and all process_stratum targets inherit from
process_stratum_target.  In earlier patches, many places in gdb were
converted to refer to threads by thread_info pointer instead of
ptid_t, but there are still places in gdb where we start with a
pid/tid and need to find the corresponding inferior or thread_info
objects.  So you'll see in the patch many places adding a
process_stratum_target parameter to functions that used to take only a
ptid_t.

Since each inferior has its own target stack now, we can always find
the process_stratum target for an inferior.  That is done via a
inf->process_target() convenience method.

Since each inferior has its own target stack, we need to handle the
"beneath" calls when servicing target calls.  The solution I settled
with is just to make sure to switch the current inferior to the
inferior you want before making a target call.  Not relying on global
context is just not feasible in current GDB.  Fortunately, there
aren't that many places that need to do that, because generally most
code that calls target methods already has the current context
pointing to the right inferior/thread.  Note, to emphasize -- there's
no method to "switch to this target stack".  Instead, you switch the
current inferior, and that implicitly switches the target stack.

In some spots, we need to iterate over all inferiors so that we reach
all target stacks.

Native targets are still singletons.  There's always only a single
instance of such targets.

Remote targets however, we'll have one instance per remote connection.

The exec target is still a singleton.  There's only one instance.  I
did not see the point of instanciating more than one exec_target
object.

After vfork, we need to make sure to push the exec target on the new
inferior.  See exec_on_vfork.

For type safety, functions that need a {target, ptid} pair to identify
a thread, take a process_stratum_target pointer for target parameter
instead of target_ops *.  Some shared code in gdb/nat/ also need to
gain a target pointer parameter.  This poses an issue, since gdbserver
doesn't have process_stratum_target, only target_ops.  To fix this,
this commit renames gdbserver's target_ops to process_stratum_target.
I think this makes sense.  There's no concept of target stack in
gdbserver, and gdbserver's target_ops really implements a
process_stratum-like target.

The thread and inferior iterator functions also gain
process_stratum_target parameters.  These are used to be able to
iterate over threads and inferiors of a given target.  Following usual
conventions, if the target pointer is null, then we iterate over
threads and inferiors of all targets.

I tried converting "add-inferior" to the gdb::option framework, as a
preparatory patch, but that stumbled on the fact that gdb::option does
not support file options yet, for "add-inferior -exec".  I have a WIP
patchset that adds that, but it's not a trivial patch, mainly due to
need to integrate readline's filename completion, so I deferred that
to some other time.

In infrun.c/infcmd.c, the main change is that we need to poll events
out of all targets.  See do_target_wait.  Right after collecting an
event, we switch the current inferior to an inferior bound to the
target that reported the event, so that target methods can be used
while handling the event.  This makes most of the code transparent to
multi-targets.  See fetch_inferior_event.

infrun.c:stop_all_threads is interesting -- in this function we need
to stop all threads of all targets.  What the function does is send an
asynchronous stop request to all threads, and then synchronously waits
for events, with target_wait, rinse repeat, until all it finds are
stopped threads.  Now that we have multiple targets, it's not
efficient to synchronously block in target_wait waiting for events out
of one target.  Instead, we implement a mini event loop, with
interruptible_select, select'ing on one file descriptor per target.
For this to work, we need to be able to ask the target for a waitable
file descriptor.  Such file descriptors already exist, they are the
descriptors registered in the main event loop with add_file_handler,
inside the target_async implementations.  This commit adds a new
target_async_wait_fd target method that just returns the file
descriptor in question.  See wait_one / stop_all_threads in infrun.c.

The 'threads_executing' global is made a per-target variable.  Since
it is only relevant to process_stratum_target targets, this is where
it is put, instead of in target_ops.

You'll notice that remote.c includes some FIXME notes.  These refer to
the fact that the global arrays that hold data for the remote packets
supported are still globals.  For example, if we connect to two
different servers/stubs, then each might support different remote
protocol features.  They might even be different architectures, like
e.g., one ARM baremetal stub, and a x86 gdbserver, to debug a
host/controller scenario as a single program.  That isn't going to
work correctly today, because of said globals.  I'm leaving fixing
that for another pass, since it does not appear to be trivial, and I'd
rather land the base work first.  It's already useful to be able to
debug multiple instances of the same server (e.g., a distributed
cluster, where you have full control over the servers installed), so I
think as is it's already reasonable incremental progress.

Current limitations:

 - you can only resume more that one target at the same time if all
   targets support asynchronous debugging, and support non-stop mode.
   It should be possible to support mixed all-stop + non-stop
   backends, but that is left for another time.  This means that
   currently in order to do multi-target with gdbserver you need to
   issue "maint set target-non-stop on".  I would like to make that
   mode be the default, but we're not there yet.  Note that I'm
   talking about how the target backend works, only.  User-visible
   all-stop mode works just fine.

 - as explained above, connecting to different remote servers at the
   same time is likely to produce bad results if they don't support the
   exact set of RSP features.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* ada-tasks.c (print_ada_task_info): Adjust find_thread_ptid call.
	(task_command_1): Likewise.
	* amd64-fbsd-tdep.c: Include "inferior.h".
	(amd64fbsd_get_thread_local_address): Pass down target.
	* amd64-linux-nat.c (ps_get_thread_area): Use ps_prochandle
	thread's gdbarch instead of target_gdbarch.
	* break-catch-sig.c (signal_catchpoint_print_it): Adjust call to
	get_last_target_status.
	* break-catch-syscall.c (print_it_catch_syscall): Likewise.
	* breakpoint.c (breakpoints_should_be_inserted_now): Consider all
	inferiors.
	(update_inserted_breakpoint_locations): Skip if inferiors with no
	execution.
	(update_global_location_list): When handling moribund locations,
	find representative inferior for location's pspace, and use thread
	count of its process_stratum target.
	* bsd-uthread.c (bsd_uthread_target::wait): Use
	as_process_stratum_target and adjust thread_change_ptid and
	add_thread calls.
	(bsd_uthread_target::update_thread_list): Use
	as_process_stratum_target and adjust find_thread_ptid,
	thread_change_ptid and add_thread calls.
	* btrace.c (maint_btrace_packet_history_cmd): Adjust
	find_thread_ptid call.
	* corelow.c (add_to_thread_list): Adjust add_thread call.
	(core_target_open): Adjust add_thread_silent and thread_count
	calls.
	(core_target::pid_to_str): Adjust find_inferior_ptid call.
	* ctf.c (ctf_target_open): Adjust add_thread_silent call.
	* event-top.c (async_disconnect): Pop targets from all inferiors.
	* exec.c (add_target_sections): Push exec target on all inferiors
	sharing the program space.
	(remove_target_sections): Remove the exec target from all
	inferiors sharing the program space.
	(exec_on_vfork): New.
	* exec.h (exec_on_vfork): Declare.
	* fbsd-tdep.c (fbsd_corefile_thread): Adjust
	get_thread_arch_regcache call.
	* fork-child.c (gdb_startup_inferior): Pass target down to
	startup_inferior and set_executing.
	* gdbthread.h (struct process_stratum_target): Forward declare.
	(add_thread, add_thread_silent, add_thread_with_info)
	(in_thread_list): Add process_stratum_target parameter.
	(find_thread_ptid(inferior*, ptid_t)): New overload.
	(find_thread_ptid, thread_change_ptid): Add process_stratum_target
	parameter.
	(all_threads()): Delete overload.
	(all_threads, all_non_exited_threads): Add process_stratum_target
	parameter.
	(all_threads_safe): Use brace initialization.
	(thread_count): Add process_stratum_target parameter.
	(set_resumed, set_running, set_stop_requested, set_executing)
	(threads_are_executing, finish_thread_state): Add
	process_stratum_target parameter.
	(switch_to_thread): Use is_current_thread.
	* i386-fbsd-tdep.c: Include "inferior.h".
	(i386fbsd_get_thread_local_address): Pass down target.
	* inf-child.c (inf_child_target::maybe_unpush_target): Remove
	have_inferiors check.
	* inf-ptrace.c (inf_ptrace_target::create_inferior)
	(inf_ptrace_target::attach): Adjust.
	* infcall.c (run_inferior_call): Adjust.
	* infcmd.c (run_command_1): Pass target to
	scoped_finish_thread_state.
	(proceed_thread_callback): Skip inferiors with no execution.
	(continue_command): Rename 'all_threads' local to avoid hiding
	'all_threads' function.  Adjust get_last_target_status call.
	(prepare_one_step): Adjust set_running call.
	(signal_command): Use user_visible_resume_target.  Compare thread
	pointers instead of inferior_ptid.
	(info_program_command): Adjust to pass down target.
	(attach_command): Mark target's 'thread_executing' flag.
	(stop_current_target_threads_ns): New, factored out from ...
	(interrupt_target_1): ... this.  Switch inferior before making
	target calls.
	* inferior-iter.h
	(struct all_inferiors_iterator, struct all_inferiors_range)
	(struct all_inferiors_safe_range)
	(struct all_non_exited_inferiors_range): Filter on
	process_stratum_target too.
	* inferior.c (inferior::inferior): Push dummy target on target
	stack.
	(find_inferior_pid, find_inferior_ptid, number_of_live_inferiors):
	Add process_stratum_target parameter, and pass it down.
	(have_live_inferiors): Adjust.
	(switch_to_inferior_and_push_target): New.
	(add_inferior_command, clone_inferior_command): Handle
	"-no-connection" parameter.  Use
	switch_to_inferior_and_push_target.
	(_initialize_inferior): Mention "-no-connection" option in
	the help of "add-inferior" and "clone-inferior" commands.
	* inferior.h: Include "process-stratum-target.h".
	(interrupt_target_1): Use bool.
	(struct inferior) <push_target, unpush_target, target_is_pushed,
	find_target_beneath, top_target, process_target, target_at,
	m_stack>: New.
	(discard_all_inferiors): Delete.
	(find_inferior_pid, find_inferior_ptid, number_of_live_inferiors)
	(all_inferiors, all_non_exited_inferiors): Add
	process_stratum_target parameter.
	* infrun.c: Include "gdb_select.h".
	(target_last_proc_target): New global.
	(follow_fork_inferior): Push target on new inferior.  Pass target
	to add_thread_silent.  Call exec_on_vfork.  Handle target's
	reference count.
	(follow_fork): Adjust get_last_target_status call.  Also consider
	target.
	(follow_exec): Push target on new inferior.
	(struct execution_control_state) <target>: New field.
	(user_visible_resume_target): New.
	(resume_1): Set target's threads_executing flag.  Consider resume
	target.
	(commit_resume_all_targets): New.
	(proceed): Also consider resume target.  Skip threads of inferiors
	with no execution.  Commit resumtion in all targets.
	(start_remote): Pass current inferior to wait_for_inferior.
	(infrun_thread_stop_requested): Consider target as well.  Pass
	thread_info pointer to clear_inline_frame_state instead of ptid.
	(infrun_thread_thread_exit): Consider target as well.
	(random_pending_event_thread): New inferior parameter.  Use it.
	(do_target_wait): Rename to ...
	(do_target_wait_1): ... this.  Add inferior parameter, and pass it
	down.
	(threads_are_resumed_pending_p, do_target_wait): New.
	(prepare_for_detach): Adjust calls.
	(wait_for_inferior): New inferior parameter.  Handle it.  Use
	do_target_wait_1 instead of do_target_wait.
	(fetch_inferior_event): Adjust.  Switch to representative
	inferior.  Pass target down.
	(set_last_target_status): Add process_stratum_target parameter.
	Save target in global.
	(get_last_target_status): Add process_stratum_target parameter and
	handle it.
	(nullify_last_target_wait_ptid): Clear 'target_last_proc_target'.
	(context_switch): Check inferior_ptid == null_ptid before calling
	inferior_thread().
	(get_inferior_stop_soon): Pass down target.
	(wait_one): Rename to ...
	(poll_one_curr_target): ... this.
	(struct wait_one_event): New.
	(wait_one): New.
	(stop_all_threads): Adjust.
	(handle_no_resumed, handle_inferior_event): Adjust to consider the
	event's target.
	(switch_back_to_stepped_thread): Also consider target.
	(print_stop_event): Update.
	(normal_stop): Update.  Also consider the resume target.
	* infrun.h (wait_for_inferior): Remove declaration.
	(user_visible_resume_target): New declaration.
	(get_last_target_status, set_last_target_status): New
	process_stratum_target parameter.
	* inline-frame.c (clear_inline_frame_state(ptid_t)): Add
	process_stratum_target parameter, and use it.
	(clear_inline_frame_state (thread_info*)): New.
	* inline-frame.c (clear_inline_frame_state(ptid_t)): Add
	process_stratum_target parameter.
	(clear_inline_frame_state (thread_info*)): Declare.
	* linux-fork.c (delete_checkpoint_command): Pass target down to
	find_thread_ptid.
	(checkpoint_command): Adjust.
	* linux-nat.c (linux_nat_target::follow_fork): Switch to thread
	instead of just tweaking inferior_ptid.
	(linux_nat_switch_fork): Pass target down to thread_change_ptid.
	(exit_lwp): Pass target down to find_thread_ptid.
	(attach_proc_task_lwp_callback): Pass target down to
	add_thread/set_running/set_executing.
	(linux_nat_target::attach): Pass target down to
	thread_change_ptid.
	(get_detach_signal): Pass target down to find_thread_ptid.
	Consider last target status's target.
	(linux_resume_one_lwp_throw, resume_lwp)
	(linux_handle_syscall_trap, linux_handle_extended_wait, wait_lwp)
	(stop_wait_callback, save_stop_reason, linux_nat_filter_event)
	(linux_nat_wait_1, resume_stopped_resumed_lwps): Pass target down.
	(linux_nat_target::async_wait_fd): New.
	(linux_nat_stop_lwp, linux_nat_target::thread_address_space): Pass
	target down.
	* linux-nat.h (linux_nat_target::async_wait_fd): Declare.
	* linux-tdep.c (get_thread_arch_regcache): Pass target down.
	* linux-thread-db.c (struct thread_db_info::process_target): New
	field.
	(add_thread_db_info): Save target.
	(get_thread_db_info): New process_stratum_target parameter.  Also
	match target.
	(delete_thread_db_info): New process_stratum_target parameter.
	Also match target.
	(thread_from_lwp): Adjust to pass down target.
	(thread_db_notice_clone): Pass down target.
	(check_thread_db_callback): Pass down target.
	(try_thread_db_load_1): Always push the thread_db target.
	(try_thread_db_load, record_thread): Pass target down.
	(thread_db_target::detach): Pass target down.  Always unpush the
	thread_db target.
	(thread_db_target::wait, thread_db_target::mourn_inferior): Pass
	target down.  Always unpush the thread_db target.
	(find_new_threads_callback, thread_db_find_new_threads_2)
	(thread_db_target::update_thread_list): Pass target down.
	(thread_db_target::pid_to_str): Pass current inferior down.
	(thread_db_target::get_thread_local_address): Pass target down.
	(thread_db_target::resume, maintenance_check_libthread_db): Pass
	target down.
	* ppc-fbsd-tdep.c: Include "inferior.h".
	(ppcfbsd_get_thread_local_address): Pass down target.
	* proc-service.c (ps_xfer_memory): Switch current inferior and
	program space as well.
	(get_ps_regcache): Pass target down.
	* process-stratum-target.c
	(process_stratum_target::thread_address_space)
	(process_stratum_target::thread_architecture): Pass target down.
	* process-stratum-target.h
	(process_stratum_target::threads_executing): New field.
	(as_process_stratum_target): New.
	* ravenscar-thread.c
	(ravenscar_thread_target::update_inferior_ptid): Pass target down.
	(ravenscar_thread_target::wait, ravenscar_add_thread): Pass target
	down.
	* record-btrace.c (record_btrace_target::info_record): Adjust.
	(record_btrace_target::record_method)
	(record_btrace_target::record_is_replaying)
	(record_btrace_target::fetch_registers)
	(get_thread_current_frame_id, record_btrace_target::resume)
	(record_btrace_target::wait, record_btrace_target::stop): Pass
	target down.
	* record-full.c (record_full_wait_1): Switch to event thread.
	Pass target down.
	* regcache.c (regcache::regcache)
	(get_thread_arch_aspace_regcache, get_thread_arch_regcache): Add
	process_stratum_target parameter and handle it.
	(current_thread_target): New global.
	(get_thread_regcache): Add process_stratum_target parameter and
	handle it.  Switch inferior before calling target method.
	(get_thread_regcache): Pass target down.
	(get_thread_regcache_for_ptid): Pass target down.
	(registers_changed_ptid): Add process_stratum_target parameter and
	handle it.
	(registers_changed_thread, registers_changed): Pass target down.
	(test_get_thread_arch_aspace_regcache): New.
	(current_regcache_test): Define a couple local test_target_ops
	instances and use them for testing.
	(readwrite_regcache): Pass process_stratum_target parameter.
	(cooked_read_test, cooked_write_test): Pass mock_target down.
	* regcache.h (get_thread_regcache, get_thread_arch_regcache)
	(get_thread_arch_aspace_regcache): Add process_stratum_target
	parameter.
	(regcache::target): New method.
	(regcache::regcache, regcache::get_thread_arch_aspace_regcache)
	(regcache::registers_changed_ptid): Add process_stratum_target
	parameter.
	(regcache::m_target): New field.
	(registers_changed_ptid): Add process_stratum_target parameter.
	* remote.c (remote_state::supports_vCont_probed): New field.
	(remote_target::async_wait_fd): New method.
	(remote_unpush_and_throw): Add remote_target parameter.
	(get_current_remote_target): Adjust.
	(remote_target::remote_add_inferior): Push target.
	(remote_target::remote_add_thread)
	(remote_target::remote_notice_new_inferior)
	(get_remote_thread_info): Pass target down.
	(remote_target::update_thread_list): Skip threads of inferiors
	bound to other targets.  (remote_target::close): Don't discard
	inferiors.  (remote_target::add_current_inferior_and_thread)
	(remote_target::process_initial_stop_replies)
	(remote_target::start_remote)
	(remote_target::remote_serial_quit_handler): Pass down target.
	(remote_target::remote_unpush_target): New remote_target
	parameter.  Unpush the target from all inferiors.
	(remote_target::remote_unpush_and_throw): New remote_target
	parameter.  Pass it down.
	(remote_target::open_1): Check whether the current inferior has
	execution instead of checking whether any inferior is live.  Pass
	target down.
	(remote_target::remote_detach_1): Pass down target.  Use
	remote_unpush_target.
	(extended_remote_target::attach): Pass down target.
	(remote_target::remote_vcont_probe): Set supports_vCont_probed.
	(remote_target::append_resumption): Pass down target.
	(remote_target::append_pending_thread_resumptions)
	(remote_target::remote_resume_with_hc, remote_target::resume)
	(remote_target::commit_resume): Pass down target.  Enable async.
	(remote_target::remote_stop_ns): Check supports_vCont_probed.
	(remote_target::interrupt_query)
	(remote_target::remove_new_fork_children)
	(remote_target::check_pending_events_prevent_wildcard_vcont)
	(remote_target::remote_parse_stop_reply)
	(remote_target::process_stop_reply): Pass down target.
	(first_remote_resumed_thread): New remote_target parameter.  Pass
	it down.
	(remote_target::wait_as): Pass down target.
	(unpush_and_perror): New remote_target parameter.  Pass it down.
	(remote_target::readchar, remote_target::remote_serial_write)
	(remote_target::getpkt_or_notif_sane_1)
	(remote_target::kill_new_fork_children, remote_target::kill): Pass
	down target.
	(remote_target::mourn_inferior): Pass down target.  Use
	remote_unpush_target.
	(remote_target::core_of_thread)
	(remote_target::remote_btrace_maybe_reopen): Pass down target.
	(remote_target::pid_to_exec_file)
	(remote_target::thread_handle_to_thread_info): Pass down target.
	(remote_target::async_wait_fd): New.
	* riscv-fbsd-tdep.c: Include "inferior.h".
	(riscv_fbsd_get_thread_local_address): Pass down target.
	* sol2-tdep.c (sol2_core_pid_to_str): Pass down target.
	* solib-spu.c (spu_skip_standalone_loader): Pass down target.
	* solib-svr4.c (enable_break): Pass down target.
	* spu-multiarch.c (parse_spufs_run): Pass down target.
	* spu-tdep.c (spu2ppu_sniffer): Pass down target.
	* target-delegates.c: Regenerate.
	* target.c (g_target_stack): Delete.
	(current_top_target): Return the current inferior's top target.
	(target_has_execution_1): Refer to the passed-in inferior's top
	target.
	(target_supports_terminal_ours): Check whether the initial
	inferior was already created.
	(decref_target): New.
	(target_stack::push): Incref/decref the target.
	(push_target, push_target, unpush_target): Adjust.
	(target_stack::unpush): Defref target.
	(target_is_pushed): Return bool.  Adjust to refer to the current
	inferior's target stack.
	(dispose_inferior): Remove 'args' parameter.  Return void.
	(target_preopen): Only dispose_inferior of the current inferior.
	(target_detach): Hold strong target reference while detaching.
	Pass target down.
	(target_thread_name): Add assertion.
	(target_resume): Pass down target.
	(target_ops::beneath, find_target_at): Adjust to refer to the
	current inferior's target stack.
	(get_dummy_target): New.
	(target_pass_ctrlc): Pass the Ctrl-C to the first inferior that
	has a thread running.
	(initialize_targets): Rename to ...
	(_initialize_target): ... this.
	* target.h: Include "gdbsupport/refcounted-object.h".
	(struct target_ops): Inherit refcounted_object.
	(target_ops::shortname, target_ops::longname): Make const.
	(target_ops::async_wait_fd): New method.
	(target_ops_ref): New typedef.
	(get_dummy_target): Declare function.
	(target_is_pushed): Return bool.
	* thread-iter.c (all_matching_threads_iterator::m_inf_matches)
	(all_matching_threads_iterator::all_matching_threads_iterator):
	Handle filter target.
	* thread-iter.h (struct all_matching_threads_iterator, struct
	all_matching_threads_range, class all_non_exited_threads_range):
	Filter by target too.
	* thread.c (threads_executing): Delete.
	(inferior_thread): Pass down current inferior.
	(clear_thread_inferior_resources): Pass down thread pointer
	instead of ptid_t.
	(add_thread_silent, add_thread_with_info, add_thread): Add
	process_stratum_target parameter.  Use it for thread and inferior
	searches.
	(is_current_thread): New.
	(thread_info::deletable): Use it.
	(find_thread_ptid, thread_count, in_thread_list)
	(thread_change_ptid, set_resumed, set_running): New
	process_stratum_target parameter.  Pass it down.
	(set_executing): New process_stratum_target parameter.  Pass it
	down.  Adjust reference to 'threads_executing'.
	(threads_are_executing): New process_stratum_target parameter.
	Adjust reference to 'threads_executing'.
	(set_stop_requested, finish_thread_state): New
	process_stratum_target parameter.  Pass it down.
	(switch_to_thread): Also match inferior.
	(switch_to_thread): New process_stratum_target parameter.  Pass it
	down.
	(update_threads_executing): Reimplement.
	* top.c (quit_force): Pop targets from all inferior.
	(gdb_init): Don't call initialize_targets.
	* tracefile-tfile.c (tfile_target_open): Pass down target.
	* gdbsupport/common-gdbthread.h (struct process_stratum_target):
	Forward declare.
	(switch_to_thread): Add process_stratum_target parameter.
	* mi/mi-interp.c (mi_on_resume_1): Add process_stratum_target
	parameter.  Use it.
	(mi_on_resume): Pass target down.
	* nat/fork-inferior.c (startup_inferior): Add
	process_stratum_target parameter.  Pass it down.
	* nat/fork-inferior.h (startup_inferior): Add
	process_stratum_target parameter.
	* python/py-threadevent.c (py_get_event_thread): Pass target down.

gdb/gdbserver/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* fork-child.c (post_fork_inferior): Pass target down to
	startup_inferior.
	* inferiors.c (switch_to_thread): Add process_stratum_target
	parameter.
	* linux-low.c (linux_target_ops): Now a process_stratum_target.
	* remote-utils.c (prepare_resume_reply): Pass the target to
	switch_to_thread.
	* target.c (the_target): Now a process_stratum_target.
	(done_accessing_memory): Pass the target to switch_to_thread.
	(set_target_ops): Ajust to use process_stratum_target.
	* target.h (struct target_ops): Rename to ...
	(struct process_stratum_target): ... this.
	(the_target, set_target_ops): Adjust.
---
 gdb/ada-tasks.c                   |   4 +-
 gdb/amd64-fbsd-tdep.c             |   4 +-
 gdb/amd64-linux-nat.c             |   2 +-
 gdb/break-catch-sig.c             |   2 +-
 gdb/break-catch-syscall.c         |   2 +-
 gdb/breakpoint.c                  |  25 +-
 gdb/bsd-uthread.c                 |  20 +-
 gdb/btrace.c                      |   2 +-
 gdb/corelow.c                     |   8 +-
 gdb/ctf.c                         |   2 +-
 gdb/event-top.c                   |  14 +-
 gdb/exec.c                        |  51 +++-
 gdb/exec.h                        |   7 +
 gdb/fbsd-tdep.c                   |   3 +-
 gdb/fork-child.c                  |   7 +-
 gdb/gdbserver/fork-child.c        |   3 +-
 gdb/gdbserver/inferiors.c         |   2 +-
 gdb/gdbserver/linux-low.c         |   2 +-
 gdb/gdbserver/remote-utils.c      |   2 +-
 gdb/gdbserver/target.c            |   8 +-
 gdb/gdbserver/target.h            |   9 +-
 gdb/gdbsupport/common-gdbthread.h |   5 +-
 gdb/gdbthread.h                   | 127 +++++----
 gdb/i386-fbsd-tdep.c              |   4 +-
 gdb/inf-child.c                   |   2 +-
 gdb/inf-ptrace.c                  |   6 +-
 gdb/infcall.c                     |   3 +-
 gdb/infcmd.c                      | 112 +++++---
 gdb/inferior-iter.h               |  77 +++++-
 gdb/inferior.c                    |  78 ++++--
 gdb/inferior.h                    |  64 ++++-
 gdb/infrun.c                      | 563 +++++++++++++++++++++++++++++---------
 gdb/infrun.h                      |  14 +-
 gdb/inline-frame.c                |  51 ++--
 gdb/inline-frame.h                |  12 +-
 gdb/linux-fork.c                  |   4 +-
 gdb/linux-nat.c                   |  73 ++---
 gdb/linux-nat.h                   |   1 +
 gdb/linux-tdep.c                  |   3 +-
 gdb/linux-thread-db.c             | 110 ++++----
 gdb/mi/mi-interp.c                |  10 +-
 gdb/nat/fork-inferior.c           |   8 +-
 gdb/nat/fork-inferior.h           |   5 +-
 gdb/ppc-fbsd-tdep.c               |   4 +-
 gdb/proc-service.c                |  17 +-
 gdb/process-stratum-target.c      |   4 +-
 gdb/process-stratum-target.h      |  16 ++
 gdb/python/py-threadevent.c       |   4 +-
 gdb/ravenscar-thread.c            |  16 +-
 gdb/record-btrace.c               |  43 +--
 gdb/record-full.c                 |  11 +-
 gdb/regcache.c                    | 162 +++++++----
 gdb/regcache.h                    |  30 +-
 gdb/remote.c                      | 220 ++++++++-------
 gdb/riscv-fbsd-tdep.c             |   4 +-
 gdb/sol2-tdep.c                   |   2 +-
 gdb/solib-spu.c                   |   3 +-
 gdb/solib-svr4.c                  |   3 +-
 gdb/spu-multiarch.c               |   6 +-
 gdb/spu-tdep.c                    |   8 +-
 gdb/target-delegates.c            |  27 ++
 gdb/target.c                      | 146 +++++++---
 gdb/target.h                      |  16 +-
 gdb/thread-iter.c                 |  14 +-
 gdb/thread-iter.h                 |  25 +-
 gdb/thread.c                      | 139 ++++++----
 gdb/top.c                         |  17 +-
 gdb/tracefile-tfile.c             |   2 +-
 68 files changed, 1686 insertions(+), 764 deletions(-)
Tom Tromey - Sept. 11, 2019, 5:11 p.m.
>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:

Pedro> This commit adds multi-target support to GDB.  What this means is that
Pedro> with this commit, GDB can now be connected to different targets at the
Pedro> same time.  E.g., you can debug a live native process and a core dump
Pedro> at the same time, connect to multiple gdbservers, etc.

Awesome!

I read through the patch, but I can't really claim to understand how all
the parts interrelate.  However your overview made sense to me, and I
don't have any conceptual concerns; just the usual sort of thing that
there are lurking bugs.  I guess my main fear is that this will
introduce regressions... but on the other hand, I think it's important
direction for gdb, so I'd rather err on the side of moving forward with
it.

Pedro> To fix this,
Pedro> this commit renames gdbserver's target_ops to process_stratum_target.
Pedro> I think this makes sense.  There's no concept of target stack in
Pedro> gdbserver, and gdbserver's target_ops really implements a
Pedro> process_stratum-like target.

Makes sense.  Sometimes I dream about re-merging the target stacks,
further shrinking the size of the gdbserver fork.

Pedro>  - you can only resume more that one target at the same time if all
Pedro>    targets support asynchronous debugging, and support non-stop mode.
Pedro>    It should be possible to support mixed all-stop + non-stop
Pedro>    backends, but that is left for another time.  This means that
Pedro>    currently in order to do multi-target with gdbserver you need to
Pedro>    issue "maint set target-non-stop on".  I would like to make that
Pedro>    mode be the default, but we're not there yet.  Note that I'm
Pedro>    talking about how the target backend works, only.  User-visible
Pedro>    all-stop mode works just fine.

Pedro>  - as explained above, connecting to different remote servers at the
Pedro>    same time is likely to produce bad results if they don't support the
Pedro>    exact set of RSP features.

Are these limitations something that can be noticed when making the
remote connection, or do they require the user to be careful?  What
happens if the user forgets or just doesn't know?

Pedro> -  thread_info *tp = find_thread_ptid (task_info->ptid);
Pedro> +  thread_info *tp = find_thread_ptid (inf->process_target (), task_info->ptid);

There are a few spots in the patch that could use the overload that
accepts an inferior, but instead call the process_target method
directly.

Pedro> +/* Calls target_commit_resume on all targets.  */
Pedro> +
Pedro> +static void
Pedro> +commit_resume_all_targets ()
Pedro> +{
Pedro> +  scoped_restore_current_thread restore_thread;
Pedro> +
Pedro> +  for (inferior *inf : all_non_exited_inferiors ())
Pedro> +    if (inf->has_execution ())
Pedro> +      {
Pedro> +	switch_to_inferior_no_thread (inf);
Pedro> +	target_commit_resume ();
Pedro> +      }

IIUC this can cause multiple calls to a given target's commit_resume
method.  That seems fine (assuming you audited these implementations)
but I suppose it would be good to document this somewhere.

Alternatively I guess gdb would need some kind of iterator that ensures
it only visits each target once.

Pedro> -	  event_ptid = wait_one (&ws);
Pedro> +	  wait_one_event event = wait_one ();
Pedro> +	  target_waitstatus &ws = event.ws;
Pedro> +	  ptid_t &event_ptid = event.ptid;

I was wondering if these need to be references.  It seemed like maybe
they could be const references but I couldn't immediately tell.

Pedro> -     For all-stop targets, we only step INFERIOR_PTID and continue others.  */
Pedro> +      For all-stop targets, we only step INFERIOR_PTID and continue others.  */

This looks like extra indentation was added.

Pedro>  struct regcache *
Pedro>  get_thread_regcache_for_ptid (ptid_t ptid)
Pedro>  {
Pedro> -  return get_thread_regcache (ptid);
Pedro> +  /* This function doesn't take a process_stratum_target parameter
Pedro> +     because it's a common/ routine implemented by both gdb and
Pedro> +     gdbserver.  It always refers to a ptid of the current target. */

It's "gdbsupport/" now.

Pedro>    /* The status of the stub support for the various vCont actions.  */
Pedro>    vCont_action_support supports_vCont;
Pedro> +  /* Whether vCont support was probed already.  */
Pedro> +  bool supports_vCont_probed;

If it's the case that this is only a temporary measure, then I think
this comment should mention that.

Pedro> @@ -6601,6 +6622,8 @@ remote_target::commit_resume ()
Pedro>      }
 
Pedro>    vcont_builder.flush ();
Pedro> +
Pedro> +  target_async (1);
Pedro>  }

I didn't understand this.  Like, is it always ok to do this?

Pedro>  /* Callback for iterate_over_inferiors.  Gets rid of the given
Pedro>     inferior.  */
 
Pedro> -static int
Pedro> -dispose_inferior (struct inferior *inf, void *args)
Pedro> +static void
Pedro> +dispose_inferior (inferior *inf)
Pedro>  {
Pedro>    /* Not all killed inferiors can, or will ever be, removed from the
Pedro>       inferior list.  Killed inferiors clearly don't need to be killed
Pedro>       again, so, we're done.  */
Pedro>    if (inf->pid == 0)
Pedro> -    return 0;
Pedro> +    return;

I think the comments here (both the intro comment and the one in the
function) need to be updated, since it seems that just a single inferior
is handled here now, and this is no longer a callback for
iterate_over_inferiors.

I didn't understand this change.  Why did it used to iterate but now
does not?

Pedro>  target_pass_ctrlc (void)
Pedro>  {
Pedro> -  current_top_target ()->pass_ctrlc ();
Pedro> +  /* Pass the Ctrl-C to the first inferior that has a thread
Pedro> +     running.  */
Pedro> +  for (inferior *inf : all_inferiors ())
Pedro> +    {
[...]
Pedro> +	      return;

I didn't understand this.  It seemed to me that a C-c should maybe be
sent to all running inferiors?

I don't actually know this area.  Maybe that's obvious now :)

Pedro>  /* See target.h.  */
Pedro> @@ -3987,10 +4047,8 @@ set_write_memory_permission (const char *args, int from_tty,
Pedro>  }
 
Pedro>  void
Pedro> -initialize_targets (void)
Pedro> +_initialize_target (void)

You might as well remove the 'void' when touching the line.

Pedro> -  explicit all_matching_threads_iterator (ptid_t filter_ptid);
Pedro> +  explicit all_matching_threads_iterator (process_stratum_target *filter_target,
Pedro> +					  ptid_t filter_ptid);

I guess this could drop the "explicit".

Pedro> -  explicit all_matching_threads_range (ptid_t filter_ptid)
Pedro> -    : m_filter_ptid (filter_ptid)
Pedro> +  explicit all_matching_threads_range (process_stratum_target *filter_target,
Pedro> +				       ptid_t filter_ptid)

Here too.

Pedro> -  explicit all_non_exited_threads_range (ptid_t filter_ptid)
Pedro> -    : m_filter_ptid (filter_ptid)
Pedro> +  explicit all_non_exited_threads_range (process_stratum_target *filter_target,
Pedro> +					 ptid_t filter_ptid)
Pedro> +    : m_filter_target (filter_target), m_filter_ptid (filter_ptid)

And here.

Tom

Patch

diff --git a/gdb/ada-tasks.c b/gdb/ada-tasks.c
index 99458aadb2..ab53f670d2 100644
--- a/gdb/ada-tasks.c
+++ b/gdb/ada-tasks.c
@@ -1095,7 +1095,7 @@  print_ada_task_info (struct ui_out *uiout,
       /* Print the associated Thread ID.  */
       if (uiout->is_mi_like_p ())
         {
-	  thread_info *thread = find_thread_ptid (task_info->ptid);
+	  thread_info *thread = find_thread_ptid (inf, task_info->ptid);
 
 	  if (thread != NULL)
 	    uiout->field_signed ("thread-id", thread->global_num);
@@ -1293,7 +1293,7 @@  task_command_1 (const char *taskno_str, int from_tty, struct inferior *inf)
      computed if target_get_ada_task_ptid has not been implemented for
      our target (yet).  Rather than cause an assertion error in that case,
      it's nicer for the user to just refuse to perform the task switch.  */
-  thread_info *tp = find_thread_ptid (task_info->ptid);
+  thread_info *tp = find_thread_ptid (inf->process_target (), task_info->ptid);
   if (tp == NULL)
     error (_("Unable to compute thread ID for task %d.\n"
              "Cannot switch to this task."),
diff --git a/gdb/amd64-fbsd-tdep.c b/gdb/amd64-fbsd-tdep.c
index 493e630974..416a34974c 100644
--- a/gdb/amd64-fbsd-tdep.c
+++ b/gdb/amd64-fbsd-tdep.c
@@ -30,6 +30,7 @@ 
 #include "amd64-tdep.h"
 #include "fbsd-tdep.h"
 #include "solib-svr4.h"
+#include "inferior.h"
 
 /* Support for signal handlers.  */
 
@@ -212,7 +213,8 @@  amd64fbsd_get_thread_local_address (struct gdbarch *gdbarch, ptid_t ptid,
 {
   struct regcache *regcache;
 
-  regcache = get_thread_arch_regcache (ptid, gdbarch);
+  regcache = get_thread_arch_regcache (current_inferior ()->process_target (),
+				       ptid, gdbarch);
 
   target_fetch_registers (regcache, AMD64_FSBASE_REGNUM);
 
diff --git a/gdb/amd64-linux-nat.c b/gdb/amd64-linux-nat.c
index 4f1c98a0d1..796dc171fb 100644
--- a/gdb/amd64-linux-nat.c
+++ b/gdb/amd64-linux-nat.c
@@ -377,7 +377,7 @@  ps_err_e
 ps_get_thread_area (struct ps_prochandle *ph,
                     lwpid_t lwpid, int idx, void **base)
 {
-  if (gdbarch_bfd_arch_info (target_gdbarch ())->bits_per_word == 32)
+  if (gdbarch_bfd_arch_info (ph->thread->inf->gdbarch)->bits_per_word == 32)
     {
       unsigned int base_addr;
       ps_err_e result;
diff --git a/gdb/break-catch-sig.c b/gdb/break-catch-sig.c
index 76f6db60ab..04168d4700 100644
--- a/gdb/break-catch-sig.c
+++ b/gdb/break-catch-sig.c
@@ -184,7 +184,7 @@  signal_catchpoint_print_it (bpstat bs)
   const char *signal_name;
   struct ui_out *uiout = current_uiout;
 
-  get_last_target_status (nullptr, &last);
+  get_last_target_status (nullptr, nullptr, &last);
 
   signal_name = signal_to_name_or_int (last.value.sig);
 
diff --git a/gdb/break-catch-syscall.c b/gdb/break-catch-syscall.c
index 0fce15d81b..7f61e43e13 100644
--- a/gdb/break-catch-syscall.c
+++ b/gdb/break-catch-syscall.c
@@ -185,7 +185,7 @@  print_it_catch_syscall (bpstat bs)
   struct syscall s;
   struct gdbarch *gdbarch = bs->bp_location_at->gdbarch;
 
-  get_last_target_status (nullptr, &last);
+  get_last_target_status (nullptr, nullptr, &last);
 
   get_syscall_by_number (gdbarch, last.value.syscall_number, &s);
 
diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 9478d2dcf2..0ab27eb611 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -390,7 +390,7 @@  breakpoints_should_be_inserted_now (void)
 	 no threads under GDB's control yet.  */
       return 1;
     }
-  else if (target_has_execution)
+  else
     {
       if (always_inserted_mode)
 	{
@@ -399,8 +399,10 @@  breakpoints_should_be_inserted_now (void)
 	  return 1;
 	}
 
-      if (threads_are_executing ())
-	return 1;
+      for (inferior *inf : all_inferiors ())
+	if (inf->has_execution ()
+	    && threads_are_executing (inf->process_target ()))
+	  return 1;
 
       /* Don't remove breakpoints yet if, even though all threads are
 	 stopped, we still have events to process.  */
@@ -2888,7 +2890,7 @@  update_inserted_breakpoint_locations (void)
 	 if we aren't attached to any process yet, we should still
 	 insert breakpoints.  */
       if (!gdbarch_has_global_breakpoints (target_gdbarch ())
-	  && inferior_ptid == null_ptid)
+	  && (inferior_ptid == null_ptid || !target_has_execution))
 	continue;
 
       val = insert_bp_location (bl, &tmp_error_stream, &disabled_breaks,
@@ -2944,7 +2946,7 @@  insert_breakpoint_locations (void)
 	 if we aren't attached to any process yet, we should still
 	 insert breakpoints.  */
       if (!gdbarch_has_global_breakpoints (target_gdbarch ())
-	  && inferior_ptid == null_ptid)
+	  && (inferior_ptid == null_ptid || !target_has_execution))
 	continue;
 
       val = insert_bp_location (bl, &tmp_error_stream, &disabled_breaks,
@@ -11880,7 +11882,18 @@  update_global_location_list (enum ugll_insert_mode insert_mode)
 		 around.  We simply always ignore hardware watchpoint
 		 traps we can no longer explain.  */
 
-	      old_loc->events_till_retirement = 3 * (thread_count () + 1);
+	      process_stratum_target *proc_target = nullptr;
+	      for (inferior *inf : all_inferiors ())
+		if (inf->pspace == old_loc->pspace)
+		  {
+		    proc_target = inf->process_target ();
+		    break;
+		  }
+	      if (proc_target != nullptr)
+		old_loc->events_till_retirement
+		  = 3 * (thread_count (proc_target) + 1);
+	      else
+		old_loc->events_till_retirement = 1;
 	      old_loc->owner = NULL;
 
 	      moribund_locations.push_back (old_loc);
diff --git a/gdb/bsd-uthread.c b/gdb/bsd-uthread.c
index af048f7a18..cdf8e2cb9c 100644
--- a/gdb/bsd-uthread.c
+++ b/gdb/bsd-uthread.c
@@ -381,9 +381,11 @@  bsd_uthread_target::wait (ptid_t ptid, struct target_waitstatus *status,
 {
   enum bfd_endian byte_order = gdbarch_byte_order (target_gdbarch ());
   CORE_ADDR addr;
+  process_stratum_target *beneath
+    = as_process_stratum_target (this->beneath ());
 
   /* Pass the request to the layer beneath.  */
-  ptid = beneath ()->wait (ptid, status, options);
+  ptid = beneath->wait (ptid, status, options);
 
   /* If the process is no longer alive, there's no point in figuring
      out the thread ID.  It will fail anyway.  */
@@ -414,13 +416,13 @@  bsd_uthread_target::wait (ptid_t ptid, struct target_waitstatus *status,
      ptid with tid set, then ptid is still the initial thread of
      the process.  Notify GDB core about it.  */
   if (inferior_ptid.tid () == 0
-      && ptid.tid () != 0 && !in_thread_list (ptid))
-    thread_change_ptid (inferior_ptid, ptid);
+      && ptid.tid () != 0 && !in_thread_list (beneath, ptid))
+    thread_change_ptid (beneath, inferior_ptid, ptid);
 
   /* Don't let the core see a ptid without a corresponding thread.  */
-  thread_info *thread = find_thread_ptid (ptid);
+  thread_info *thread = find_thread_ptid (beneath, ptid);
   if (thread == NULL || thread->state == THREAD_EXITED)
-    add_thread (ptid);
+    add_thread (beneath, ptid);
 
   return ptid;
 }
@@ -467,16 +469,18 @@  bsd_uthread_target::update_thread_list ()
     {
       ptid_t ptid = ptid_t (pid, 0, addr);
 
-      thread_info *thread = find_thread_ptid (ptid);
+      process_stratum_target *proc_target
+	= as_process_stratum_target (this->beneath ());
+      thread_info *thread = find_thread_ptid (proc_target, ptid);
       if (thread == nullptr || thread->state == THREAD_EXITED)
 	{
 	  /* If INFERIOR_PTID doesn't have a tid member yet, then ptid
 	     is still the initial thread of the process.  Notify GDB
 	     core about it.  */
 	  if (inferior_ptid.tid () == 0)
-	    thread_change_ptid (inferior_ptid, ptid);
+	    thread_change_ptid (proc_target, inferior_ptid, ptid);
 	  else
-	    add_thread (ptid);
+	    add_thread (proc_target, ptid);
 	}
 
       addr = bsd_uthread_read_memory_address (addr + offset);
diff --git a/gdb/btrace.c b/gdb/btrace.c
index 466607dbfc..303ac1f2c4 100644
--- a/gdb/btrace.c
+++ b/gdb/btrace.c
@@ -3237,7 +3237,7 @@  maint_btrace_packet_history_cmd (const char *arg, int from_tty)
   struct btrace_thread_info *btinfo;
   unsigned int size, begin, end, from, to;
 
-  thread_info *tp = find_thread_ptid (inferior_ptid);
+  thread_info *tp = find_thread_ptid (current_inferior (), inferior_ptid);
   if (tp == NULL)
     error (_("No thread."));
 
diff --git a/gdb/corelow.c b/gdb/corelow.c
index 0fba93f802..d98713fc37 100644
--- a/gdb/corelow.c
+++ b/gdb/corelow.c
@@ -312,7 +312,7 @@  add_to_thread_list (bfd *abfd, asection *asect, void *reg_sect_arg)
 
   ptid = ptid_t (pid, lwpid, 0);
 
-  add_thread (ptid);
+  add_thread (inf->process_target (), ptid);
 
 /* Warning, Will Robinson, looking at BFD private data! */
 
@@ -450,7 +450,7 @@  core_target_open (const char *arg, int from_tty)
 	{
 	  inferior_appeared (current_inferior (), CORELOW_PID);
 	  inferior_ptid = ptid_t (CORELOW_PID);
-	  add_thread_silent (inferior_ptid);
+	  add_thread_silent (target, inferior_ptid);
 	}
       else
 	switch_to_thread (thread);
@@ -515,7 +515,7 @@  core_target_open (const char *arg, int from_tty)
   /* Current thread should be NUM 1 but the user does not know that.
      If a program is single threaded gdb in general does not mention
      anything about threads.  That is why the test is >= 2.  */
-  if (thread_count () >= 2)
+  if (thread_count (target) >= 2)
     {
       try
 	{
@@ -1008,7 +1008,7 @@  core_target::pid_to_str (ptid_t ptid)
 
   /* Otherwise, this isn't a "threaded" core -- use the PID field, but
      only if it isn't a fake PID.  */
-  inf = find_inferior_ptid (ptid);
+  inf = find_inferior_ptid (this, ptid);
   if (inf != NULL && !inf->fake_pid_p)
     return normal_pid_to_str (ptid);
 
diff --git a/gdb/ctf.c b/gdb/ctf.c
index b3c3f0d7b2..c17bd1bbe2 100644
--- a/gdb/ctf.c
+++ b/gdb/ctf.c
@@ -1169,7 +1169,7 @@  ctf_target_open (const char *dirname, int from_tty)
 
   inferior_appeared (current_inferior (), CTF_PID);
   inferior_ptid = ptid_t (CTF_PID);
-  add_thread_silent (inferior_ptid);
+  add_thread_silent (&ctf_ops, inferior_ptid);
 
   merge_uploaded_trace_state_variables (&uploaded_tsvs);
   merge_uploaded_tracepoints (&uploaded_tps);
diff --git a/gdb/event-top.c b/gdb/event-top.c
index 2132fb550d..635270cc90 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -1095,12 +1095,16 @@  async_disconnect (gdb_client_data arg)
       exception_print (gdb_stderr, exception);
     }
 
-  try
-    {
-      pop_all_targets ();
-    }
-  catch (const gdb_exception &exception)
+  for (inferior *inf : all_inferiors ())
     {
+      switch_to_inferior_no_thread (inf);
+      try
+	{
+	  pop_all_targets ();
+	}
+      catch (const gdb_exception &exception)
+	{
+	}
     }
 
   signal (SIGHUP, SIG_DFL);	/*FIXME: ???????????  */
diff --git a/gdb/exec.c b/gdb/exec.c
index 3098fcaadd..53d6303160 100644
--- a/gdb/exec.c
+++ b/gdb/exec.c
@@ -547,10 +547,23 @@  add_target_sections (void *owner,
 	  table->sections[space + i].owner = owner;
 	}
 
+      scoped_restore_current_thread restore_thread;
+      program_space *curr_pspace = current_program_space;
+
       /* If these are the first file sections we can provide memory
-	 from, push the file_stratum target.  */
-      if (!target_is_pushed (&exec_ops))
-	push_target (&exec_ops);
+	 from, push the file_stratum target.  Must do this in all
+	 inferiors sharing the program space.  */
+      for (inferior *inf : all_inferiors ())
+	{
+	  if (inf->pspace != curr_pspace)
+	    continue;
+
+	  if (inf->target_is_pushed (&exec_ops))
+	    continue;
+
+	  switch_to_inferior_no_thread (inf);
+	  push_target (&exec_ops);
+	}
     }
 }
 
@@ -628,21 +641,39 @@  remove_target_sections (void *owner)
       old_count = resize_section_table (table, dest - src);
 
       /* If we don't have any more sections to read memory from,
-	 remove the file_stratum target from the stack.  */
+	 remove the file_stratum target from the stack of each
+	 inferior sharing the program space.  */
       if (old_count + (dest - src) == 0)
 	{
-	  struct program_space *pspace;
+	  scoped_restore_current_thread restore_thread;
+	  program_space *curr_pspace = current_program_space;
+
+	  for (inferior *inf : all_inferiors ())
+	    {
+	      if (inf->pspace != curr_pspace)
+		continue;
 
-	  ALL_PSPACES (pspace)
-	    if (pspace->target_sections.sections
-		!= pspace->target_sections.sections_end)
-	      return;
+	      if (inf->pspace->target_sections.sections
+		  != inf->pspace->target_sections.sections_end)
+		continue;
 
-	  unpush_target (&exec_ops);
+	      switch_to_inferior_no_thread (inf);
+	      unpush_target (&exec_ops);
+	    }
 	}
     }
 }
 
+/* See exec.h.  */
+
+void
+exec_on_vfork ()
+{
+  if (current_program_space->target_sections.sections
+      != current_program_space->target_sections.sections_end)
+    push_target (&exec_ops);
+}
+
 
 
 enum target_xfer_status
diff --git a/gdb/exec.h b/gdb/exec.h
index e9af480287..539e33f417 100644
--- a/gdb/exec.h
+++ b/gdb/exec.h
@@ -44,6 +44,13 @@  extern int build_section_table (struct bfd *, struct target_section **,
 
 extern void clear_section_table (struct target_section_table *table);
 
+/* The current inferior is a child vforked and its program space is
+   shared with its parent.  This pushes the exec target on the
+   current/child inferior's target stack if there are sections in the
+   program space's section table.  */
+
+extern void exec_on_vfork ();
+
 /* Read from mappable read-only sections of BFD executable files.
    Return TARGET_XFER_OK, if read is successful.  Return
    TARGET_XFER_EOF if read is done.  Return TARGET_XFER_E_IO
diff --git a/gdb/fbsd-tdep.c b/gdb/fbsd-tdep.c
index e81c6c99bf..182494f561 100644
--- a/gdb/fbsd-tdep.c
+++ b/gdb/fbsd-tdep.c
@@ -673,7 +673,8 @@  fbsd_corefile_thread (struct thread_info *info,
 {
   struct regcache *regcache;
 
-  regcache = get_thread_arch_regcache (info->ptid, args->gdbarch);
+  regcache = get_thread_arch_regcache (info->inf->process_target (),
+				       info->ptid, args->gdbarch);
 
   target_fetch_registers (regcache, -1);
 
diff --git a/gdb/fork-child.c b/gdb/fork-child.c
index edfb6a3c5a..384e45b2e0 100644
--- a/gdb/fork-child.c
+++ b/gdb/fork-child.c
@@ -128,10 +128,13 @@  postfork_child_hook ()
 ptid_t
 gdb_startup_inferior (pid_t pid, int num_traps)
 {
-  ptid_t ptid = startup_inferior (pid, num_traps, NULL, NULL);
+  inferior *inf = current_inferior ();
+  process_stratum_target *proc_target = inf->process_target ();
+
+  ptid_t ptid = startup_inferior (proc_target, pid, num_traps, NULL, NULL);
 
   /* Mark all threads non-executing.  */
-  set_executing (ptid, 0);
+  set_executing (proc_target, ptid, 0);
 
   return ptid;
 }
diff --git a/gdb/gdbserver/fork-child.c b/gdb/gdbserver/fork-child.c
index ddd73094e6..24b31320fd 100644
--- a/gdb/gdbserver/fork-child.c
+++ b/gdb/gdbserver/fork-child.c
@@ -107,7 +107,8 @@  post_fork_inferior (int pid, const char *program)
   atexit (restore_old_foreground_pgrp);
 #endif
 
-  startup_inferior (pid, START_INFERIOR_TRAPS_EXPECTED,
+  startup_inferior (the_target, pid,
+		    START_INFERIOR_TRAPS_EXPECTED,
 		    &cs.last_status, &cs.last_ptid);
   current_thread->last_resume_kind = resume_stop;
   current_thread->last_status = cs.last_status;
diff --git a/gdb/gdbserver/inferiors.c b/gdb/gdbserver/inferiors.c
index e35186bda1..09a98a9145 100644
--- a/gdb/gdbserver/inferiors.c
+++ b/gdb/gdbserver/inferiors.c
@@ -216,7 +216,7 @@  current_process (void)
 /* See gdbsupport/common-gdbthread.h.  */
 
 void
-switch_to_thread (ptid_t ptid)
+switch_to_thread (process_stratum_target *ops, ptid_t ptid)
 {
   gdb_assert (ptid != minus_one_ptid);
   current_thread = find_thread_ptid (ptid);
diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 3113017ae6..60efe46002 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -7454,7 +7454,7 @@  linux_get_hwcap2 (int wordsize)
   return hwcap2;
 }
 
-static struct target_ops linux_target_ops = {
+static process_stratum_target linux_target_ops = {
   linux_create_inferior,
   linux_post_create_inferior,
   linux_attach,
diff --git a/gdb/gdbserver/remote-utils.c b/gdb/gdbserver/remote-utils.c
index 665fc66c53..64eb7118c6 100644
--- a/gdb/gdbserver/remote-utils.c
+++ b/gdb/gdbserver/remote-utils.c
@@ -1210,7 +1210,7 @@  prepare_resume_reply (char *buf, ptid_t ptid,
 
 	saved_thread = current_thread;
 
-	switch_to_thread (ptid);
+	switch_to_thread (the_target, ptid);
 
 	regp = current_target_desc ()->expedite_regs;
 
diff --git a/gdb/gdbserver/target.c b/gdb/gdbserver/target.c
index 0b45b6c956..efd412aa69 100644
--- a/gdb/gdbserver/target.c
+++ b/gdb/gdbserver/target.c
@@ -22,7 +22,7 @@ 
 #include "tracepoint.h"
 #include "gdbsupport/byte-vector.h"
 
-struct target_ops *the_target;
+process_stratum_target *the_target;
 
 int
 set_desired_thread ()
@@ -119,7 +119,7 @@  done_accessing_memory (void)
 
   /* Restore the previous selected thread.  */
   cs.general_thread = prev_general_thread;
-  switch_to_thread (cs.general_thread);
+  switch_to_thread (the_target, cs.general_thread);
 }
 
 int
@@ -284,9 +284,9 @@  start_non_stop (int nonstop)
 }
 
 void
-set_target_ops (struct target_ops *target)
+set_target_ops (process_stratum_target *target)
 {
-  the_target = XNEW (struct target_ops);
+  the_target = XNEW (process_stratum_target);
   memcpy (the_target, target, sizeof (*the_target));
 }
 
diff --git a/gdb/gdbserver/target.h b/gdb/gdbserver/target.h
index 67167cca2d..bfa55d7244 100644
--- a/gdb/gdbserver/target.h
+++ b/gdb/gdbserver/target.h
@@ -63,7 +63,10 @@  struct thread_resume
   CORE_ADDR step_range_end;	/* Exclusive */
 };
 
-struct target_ops
+/* GDBserver doesn't have a concept of strata like GDB, but we call
+   its target vector "process_stratum" anyway for the benefit of
+   shared code.  */
+struct process_stratum_target
 {
   /* Start a new process.
 
@@ -480,9 +483,9 @@  struct target_ops
   bool (*thread_handle) (ptid_t ptid, gdb_byte **handle, int *handle_len);
 };
 
-extern struct target_ops *the_target;
+extern process_stratum_target *the_target;
 
-void set_target_ops (struct target_ops *);
+void set_target_ops (process_stratum_target *);
 
 #define create_inferior(program, program_args)	\
   (*the_target->create_inferior) (program, program_args)
diff --git a/gdb/gdbsupport/common-gdbthread.h b/gdb/gdbsupport/common-gdbthread.h
index d692be209c..20f4d89b26 100644
--- a/gdb/gdbsupport/common-gdbthread.h
+++ b/gdb/gdbsupport/common-gdbthread.h
@@ -19,7 +19,10 @@ 
 #ifndef COMMON_COMMON_GDBTHREAD_H
 #define COMMON_COMMON_GDBTHREAD_H
 
+struct process_stratum_target;
+
 /* Switch from one thread to another.  */
-extern void switch_to_thread (ptid_t ptid);
+extern void switch_to_thread (process_stratum_target *proc_target,
+			      ptid_t ptid);
 
 #endif /* COMMON_COMMON_GDBTHREAD_H */
diff --git a/gdb/gdbthread.h b/gdb/gdbthread.h
index bb7a71af0b..d00cc3fb8f 100644
--- a/gdb/gdbthread.h
+++ b/gdb/gdbthread.h
@@ -35,6 +35,7 @@  struct symtab;
 #include "gdbsupport/forward-scope-exit.h"
 
 struct inferior;
+struct process_stratum_target;
 
 /* Frontend view of the thread state.  Possible extensions: stepping,
    finishing, until(ling),...
@@ -305,7 +306,7 @@  public:
      from saying that there is an active target and we are stopped at
      a breakpoint, for instance.  This is a real indicator whether the
      thread is off and running.  */
-  int executing = 0;
+  bool executing = false;
 
   /* Non-zero if this thread is resumed from infrun's perspective.
      Note that a thread can be marked both as not-executing and
@@ -420,15 +421,18 @@  extern void init_thread_list (void);
    that a new thread is found, and return the pointer to
    the new thread.  Caller my use this pointer to 
    initialize the private thread data.  */
-extern struct thread_info *add_thread (ptid_t ptid);
+extern struct thread_info *add_thread (process_stratum_target *targ,
+				       ptid_t ptid);
 
-/* Same as add_thread, but does not print a message
-   about new thread.  */
-extern struct thread_info *add_thread_silent (ptid_t ptid);
+/* Same as add_thread, but does not print a message about new
+   thread.  */
+extern struct thread_info *add_thread_silent (process_stratum_target *targ,
+					      ptid_t ptid);
 
 /* Same as add_thread, and sets the private info.  */
-extern struct thread_info *add_thread_with_info (ptid_t ptid,
-						 struct private_thread_info *);
+extern struct thread_info *add_thread_with_info (process_stratum_target *targ,
+						 ptid_t ptid,
+						 private_thread_info *);
 
 /* Delete an existing thread list entry.  */
 extern void delete_thread (struct thread_info *thread);
@@ -469,14 +473,18 @@  extern int show_inferior_qualified_tids (void);
 const char *print_thread_id (struct thread_info *thr);
 
 /* Boolean test for an already-known ptid.  */
-extern int in_thread_list (ptid_t ptid);
+extern bool in_thread_list (process_stratum_target *targ, ptid_t ptid);
 
 /* Boolean test for an already-known global thread id (GDB's homegrown
    global id, not the system's).  */
 extern int valid_global_thread_id (int global_id);
 
+/* Find thread PTID of inferior INF.  */
+extern thread_info *find_thread_ptid (inferior *inf, ptid_t ptid);
+
 /* Search function to lookup a thread by 'pid'.  */
-extern struct thread_info *find_thread_ptid (ptid_t ptid);
+extern struct thread_info *find_thread_ptid (process_stratum_target *targ,
+					     ptid_t ptid);
 
 /* Search function to lookup a thread by 'ptid'.  Only searches in
    threads of INF.  */
@@ -501,7 +509,8 @@  extern struct thread_info *any_thread_of_inferior (inferior *inf);
 extern struct thread_info *any_live_thread_of_inferior (inferior *inf);
 
 /* Change the ptid of thread OLD_PTID to NEW_PTID.  */
-void thread_change_ptid (ptid_t old_ptid, ptid_t new_ptid);
+void thread_change_ptid (process_stratum_target *targ,
+			 ptid_t old_ptid, ptid_t new_ptid);
 
 /* Iterator function to call a user-provided callback function
    once for each known thread.  */
@@ -512,34 +521,44 @@  extern struct thread_info *iterate_over_threads (thread_callback_func, void *);
    iterators.  Must be done after struct thread_info is defined.  */
 #include "thread-iter.h"
 
-/* Return a range that can be used to walk over all threads of all
-   inferiors, with range-for.  Used like this:
+/* Return a range that can be used to walk over threads, with
+   range-for.
+
+   Used like this, it walks over all threads of all inferiors of all
+   targets:
 
        for (thread_info *thr : all_threads ())
 	 { .... }
-*/
-inline all_threads_range
-all_threads ()
-{
-  return {};
-}
 
-/* Likewise, but accept a filter PTID.  */
+   FILTER_PTID can be used to filter out threads that don't match.
+   FILTER_PTID can be:
+
+   - minus_one_ptid, meaning walk all threads of all inferiors of
+     PROC_TARGET.  If PROC_TARGET is NULL, then of all targets.
+
+   - A process ptid, in which case walk all threads of the specified
+     process.  PROC_TARGET must be non-NULL in this case.
+
+   - A thread ptid, in which case walk that thread only.  PROC_TARGET
+     must be non-NULL in this case.
+*/
 
 inline all_matching_threads_range
-all_threads (ptid_t filter_ptid)
+all_threads (process_stratum_target *proc_target = nullptr,
+	     ptid_t filter_ptid = minus_one_ptid)
 {
-  return all_matching_threads_range (filter_ptid);
+  return all_matching_threads_range (proc_target, filter_ptid);
 }
 
 /* Return a range that can be used to walk over all non-exited threads
-   of all inferiors, with range-for.  FILTER_PTID can be used to
-   filter out thread that don't match.  */
+   of all inferiors, with range-for.  Arguments are like all_threads
+   above.  */
 
 inline all_non_exited_threads_range
-all_non_exited_threads (ptid_t filter_ptid = minus_one_ptid)
+all_non_exited_threads (process_stratum_target *proc_target = nullptr,
+			ptid_t filter_ptid = minus_one_ptid)
 {
-  return all_non_exited_threads_range (filter_ptid);
+  return all_non_exited_threads_range (proc_target, filter_ptid);
 }
 
 /* Return a range that can be used to walk over all threads of all
@@ -555,10 +574,10 @@  all_non_exited_threads (ptid_t filter_ptid = minus_one_ptid)
 inline all_threads_safe_range
 all_threads_safe ()
 {
-  return all_threads_safe_range ();
+  return {};
 }
 
-extern int thread_count (void);
+extern int thread_count (process_stratum_target *proc_target);
 
 /* Return true if we have any thread in any inferior.  */
 extern bool any_thread_p ();
@@ -572,44 +591,50 @@  extern void switch_to_no_thread ();
 /* Switch from one thread to another.  Does not read registers.  */
 extern void switch_to_thread_no_regs (struct thread_info *thread);
 
-/* Marks or clears thread(s) PTID as resumed.  If PTID is
-   MINUS_ONE_PTID, applies to all threads.  If ptid_is_pid(PTID) is
-   true, applies to all threads of the process pointed at by PTID.  */
-extern void set_resumed (ptid_t ptid, int resumed);
-
-/* Marks thread PTID is running, or stopped. 
-   If PTID is minus_one_ptid, marks all threads.  */
-extern void set_running (ptid_t ptid, int running);
-
-/* Marks or clears thread(s) PTID as having been requested to stop.
-   If PTID is MINUS_ONE_PTID, applies to all threads.  If
+/* Marks or clears thread(s) PTID of TARG as resumed.  If PTID is
+   MINUS_ONE_PTID, applies to all threads of TARG.  If
    ptid_is_pid(PTID) is true, applies to all threads of the process
-   pointed at by PTID.  If STOP, then the THREAD_STOP_REQUESTED
-   observer is called with PTID as argument.  */
-extern void set_stop_requested (ptid_t ptid, int stop);
-
-/* Marks thread PTID as executing, or not.  If PTID is minus_one_ptid,
-   marks all threads.
+   pointed at by {TARG,PTID}.  */
+extern void set_resumed (process_stratum_target *targ,
+			 ptid_t ptid, bool resumed);
+
+/* Marks thread PTID of TARG as running, or as stopped.  If PTID is
+   minus_one_ptid, marks all threads of TARG.  */
+extern void set_running (process_stratum_target *targ,
+			 ptid_t ptid, bool running);
+
+/* Marks or clears thread(s) PTID of TARG as having been requested to
+   stop.  If PTID is MINUS_ONE_PTID, applies to all threads of TARG.
+   If ptid_is_pid(PTID) is true, applies to all threads of the process
+   pointed at by {TARG, PTID}.  If STOP, then the
+   THREAD_STOP_REQUESTED observer is called with PTID as argument.  */
+extern void set_stop_requested (process_stratum_target *targ,
+				ptid_t ptid, bool stop);
+
+/* Marks thread PTID of TARG as executing, or not.  If PTID is
+   minus_one_ptid, marks all threads of TARG.
 
    Note that this is different from the running state.  See the
    description of state and executing fields of struct
    thread_info.  */
-extern void set_executing (ptid_t ptid, int executing);
+extern void set_executing (process_stratum_target *targ,
+			   ptid_t ptid, bool executing);
 
-/* True if any (known or unknown) thread is or may be executing.  */
-extern int threads_are_executing (void);
+/* True if any (known or unknown) thread of TARG is or may be
+   executing.  */
+extern bool threads_are_executing (process_stratum_target *targ);
 
-/* Merge the executing property of thread PTID over to its thread
-   state property (frontend running/stopped view).
+/* Merge the executing property of thread PTID of TARG over to its
+   thread state property (frontend running/stopped view).
 
    "not executing" -> "stopped"
    "executing"     -> "running"
    "exited"        -> "exited"
 
-   If PTID is minus_one_ptid, go over all threads.
+   If PTID is minus_one_ptid, go over all threads of TARG.
 
    Notifications are only emitted if the thread state did change.  */
-extern void finish_thread_state (ptid_t ptid);
+extern void finish_thread_state (process_stratum_target *targ, ptid_t ptid);
 
 /* Calls finish_thread_state on scope exit, unless release() is called
    to disengage.  */
diff --git a/gdb/i386-fbsd-tdep.c b/gdb/i386-fbsd-tdep.c
index 3848bf890d..291a624b2c 100644
--- a/gdb/i386-fbsd-tdep.c
+++ b/gdb/i386-fbsd-tdep.c
@@ -30,6 +30,7 @@ 
 #include "i387-tdep.h"
 #include "fbsd-tdep.h"
 #include "solib-svr4.h"
+#include "inferior.h"
 
 /* Support for signal handlers.  */
 
@@ -332,7 +333,8 @@  i386fbsd_get_thread_local_address (struct gdbarch *gdbarch, ptid_t ptid,
   if (tdep->fsbase_regnum == -1)
     error (_("Unable to fetch %%gsbase"));
 
-  regcache = get_thread_arch_regcache (ptid, gdbarch);
+  regcache = get_thread_arch_regcache (current_inferior ()->process_target (),
+				       ptid, gdbarch);
 
   target_fetch_registers (regcache, tdep->fsbase_regnum + 1);
 
diff --git a/gdb/inf-child.c b/gdb/inf-child.c
index 0a73965a0b..4631e323ae 100644
--- a/gdb/inf-child.c
+++ b/gdb/inf-child.c
@@ -206,7 +206,7 @@  inf_child_target::mourn_inferior ()
 void
 inf_child_target::maybe_unpush_target ()
 {
-  if (!inf_child_explicitly_opened && !have_inferiors ())
+  if (!inf_child_explicitly_opened)
     unpush_target (this);
 }
 
diff --git a/gdb/inf-ptrace.c b/gdb/inf-ptrace.c
index 4a8e732373..1e36ce60f9 100644
--- a/gdb/inf-ptrace.c
+++ b/gdb/inf-ptrace.c
@@ -136,7 +136,7 @@  inf_ptrace_target::create_inferior (const char *exec_file,
   /* We have something that executes now.  We'll be running through
      the shell at this point (if startup-with-shell is true), but the
      pid shouldn't change.  */
-  add_thread_silent (ptid);
+  add_thread_silent (this, ptid);
 
   unpusher.release ();
 
@@ -236,10 +236,10 @@  inf_ptrace_target::attach (const char *args, int from_tty)
 
   /* Always add a main thread.  If some target extends the ptrace
      target, it should decorate the ptid later with more info.  */
-  thread_info *thr = add_thread_silent (inferior_ptid);
+  thread_info *thr = add_thread_silent (this, inferior_ptid);
   /* Don't consider the thread stopped until we've processed its
      initial SIGSTOP stop.  */
-  set_executing (thr->ptid, true);
+  set_executing (this, thr->ptid, true);
 
   unpusher.release ();
 }
diff --git a/gdb/infcall.c b/gdb/infcall.c
index 23f17ee0e2..ff7e16848e 100644
--- a/gdb/infcall.c
+++ b/gdb/infcall.c
@@ -650,7 +650,8 @@  run_inferior_call (struct call_thread_fsm *sm,
   if (!was_running
       && call_thread_ptid == inferior_ptid
       && stop_stack_dummy == STOP_STACK_DUMMY)
-    finish_thread_state (user_visible_resume_ptid (0));
+    finish_thread_state (call_thread->inf->process_target (),
+			 user_visible_resume_ptid (0));
 
   enable_watchpoints_after_interactive_call_stop ();
 
diff --git a/gdb/infcmd.c b/gdb/infcmd.c
index 879f33bbc2..f2774bc1f2 100644
--- a/gdb/infcmd.c
+++ b/gdb/infcmd.c
@@ -653,10 +653,19 @@  run_command_1 (const char *args, int from_tty, enum run_how run_how)
      events --- the frontend shouldn't see them as stopped.  In
      all-stop, always finish the state of all threads, as we may be
      resuming more than just the new process.  */
-  ptid_t finish_ptid = (non_stop
-			? ptid_t (current_inferior ()->pid)
-			: minus_one_ptid);
-  scoped_finish_thread_state finish_state (finish_ptid);
+  process_stratum_target *finish_target;
+  ptid_t finish_ptid;
+  if (non_stop)
+    {
+      finish_target = current_inferior ()->process_target ();
+      finish_ptid = ptid_t (current_inferior ()->pid);
+    }
+  else
+    {
+      finish_target = nullptr;
+      finish_ptid = minus_one_ptid;
+    }
+  scoped_finish_thread_state finish_state (finish_target, finish_ptid);
 
   /* Pass zero for FROM_TTY, because at this point the "run" command
      has done its thing; now we are setting up the running program.  */
@@ -726,6 +735,9 @@  proceed_thread_callback (struct thread_info *thread, void *arg)
   if (thread->state != THREAD_STOPPED)
     return 0;
 
+  if (!thread->inf->has_execution ())
+    return 0;
+
   switch_to_thread (thread);
   clear_proceed_status (0);
   proceed ((CORE_ADDR) -1, GDB_SIGNAL_DEFAULT);
@@ -819,7 +831,7 @@  static void
 continue_command (const char *args, int from_tty)
 {
   int async_exec;
-  int all_threads = 0;
+  bool all_threads_p = false;
 
   ERROR_NO_INFERIOR;
 
@@ -831,17 +843,17 @@  continue_command (const char *args, int from_tty)
     {
       if (startswith (args, "-a"))
 	{
-	  all_threads = 1;
+	  all_threads_p = true;
 	  args += sizeof ("-a") - 1;
 	  if (*args == '\0')
 	    args = NULL;
 	}
     }
 
-  if (!non_stop && all_threads)
+  if (!non_stop && all_threads_p)
     error (_("`-a' is meaningless in all-stop mode."));
 
-  if (args != NULL && all_threads)
+  if (args != NULL && all_threads_p)
     error (_("Can't resume all threads and specify "
 	     "proceed count simultaneously."));
 
@@ -858,10 +870,11 @@  continue_command (const char *args, int from_tty)
 	tp = inferior_thread ();
       else
 	{
+	  process_stratum_target *last_target;
 	  ptid_t last_ptid;
 
-	  get_last_target_status (&last_ptid, nullptr);
-	  tp = find_thread_ptid (last_ptid);
+	  get_last_target_status (&last_target, &last_ptid, nullptr);
+	  tp = find_thread_ptid (last_target, last_ptid);
 	}
       if (tp != NULL)
 	bs = tp->control.stop_bpstat;
@@ -889,7 +902,7 @@  continue_command (const char *args, int from_tty)
   ERROR_NO_INFERIOR;
   ensure_not_tfind_mode ();
 
-  if (!non_stop || !all_threads)
+  if (!non_stop || !all_threads_p)
     {
       ensure_valid_thread ();
       ensure_not_running ();
@@ -900,7 +913,7 @@  continue_command (const char *args, int from_tty)
   if (from_tty)
     printf_filtered (_("Continuing.\n"));
 
-  continue_1 (all_threads);
+  continue_1 (all_threads_p);
 }
 
 /* Record the starting point of a "step" or "next" command.  */
@@ -1117,7 +1130,7 @@  prepare_one_step (struct step_command_fsm *sm)
 
 	      /* Pretend that we've ran.  */
 	      resume_ptid = user_visible_resume_ptid (1);
-	      set_running (resume_ptid, 1);
+	      set_running (tp->inf->process_target (), resume_ptid, true);
 
 	      step_into_inline_frame (tp);
 	      sm->count--;
@@ -1309,10 +1322,14 @@  signal_command (const char *signum_exp, int from_tty)
       /* This indicates what will be resumed.  Either a single thread,
 	 a whole process, or all threads of all processes.  */
       ptid_t resume_ptid = user_visible_resume_ptid (0);
+      process_stratum_target *resume_target
+	= user_visible_resume_target (resume_ptid);
 
-      for (thread_info *tp : all_non_exited_threads (resume_ptid))
+      thread_info *current = inferior_thread ();
+
+      for (thread_info *tp : all_non_exited_threads (resume_target, resume_ptid))
 	{
-	  if (tp->ptid == inferior_ptid)
+	  if (tp == current)
 	    continue;
 
 	  if (tp->suspend.stop_signal != GDB_SIGNAL_0
@@ -1974,6 +1991,7 @@  info_program_command (const char *args, int from_tty)
   bpstat bs;
   int num, stat;
   ptid_t ptid;
+  process_stratum_target *proc_target;
 
   if (!target_has_execution)
     {
@@ -1982,14 +2000,17 @@  info_program_command (const char *args, int from_tty)
     }
 
   if (non_stop)
-    ptid = inferior_ptid;
+    {
+      ptid = inferior_ptid;
+      proc_target = current_inferior ()->process_target ();
+    }
   else
-    get_last_target_status (&ptid, nullptr);
+    get_last_target_status (&proc_target, &ptid, nullptr);
 
   if (ptid == null_ptid || ptid == minus_one_ptid)
     error (_("No selected thread."));
 
-  thread_info *tp = find_thread_ptid (ptid);
+  thread_info *tp = find_thread_ptid (proc_target, ptid);
 
   if (tp->state == THREAD_EXITED)
     error (_("Invalid selected thread."));
@@ -2778,12 +2799,16 @@  attach_command (const char *args, int from_tty)
       add_inferior_continuation (attach_command_continuation, a,
 				 attach_command_continuation_free_args);
 
+      /* Let infrun consider waiting for events out of this
+	 target.  */
+      inferior->process_target ()->threads_executing = true;
+
       if (!target_is_async_p ())
 	mark_infrun_async_event_handler ();
       return;
     }
-
-  attach_post_wait (args, from_tty, mode);
+  else
+    attach_post_wait (args, from_tty, mode);
 }
 
 /* We had just found out that the target was already attached to an
@@ -2900,20 +2925,15 @@  disconnect_command (const char *args, int from_tty)
     deprecated_detach_hook ();
 }
 
-void 
-interrupt_target_1 (int all_threads)
-{
-  ptid_t ptid;
-
-  if (all_threads)
-    ptid = minus_one_ptid;
-  else
-    ptid = inferior_ptid;
+/* Stop PTID in the current target, and tag the PTID threads as having
+   been explicitly requested to stop.  PTID can be a thread, a
+   process, or minus_one_ptid, meaning all threads of all inferiors of
+   the current target.  */
 
-  if (non_stop)
-    target_stop (ptid);
-  else
-    target_interrupt ();
+static void
+stop_current_target_threads_ns (ptid_t ptid)
+{
+  target_stop (ptid);
 
   /* Tag the thread as having been explicitly requested to stop, so
      other parts of gdb know not to resume this thread automatically,
@@ -2921,8 +2941,32 @@  interrupt_target_1 (int all_threads)
      non-stop mode, as when debugging a multi-threaded application in
      all-stop mode, we will only get one stop event --- it's undefined
      which thread will report the event.  */
+  set_stop_requested (current_inferior ()->process_target (),
+		      ptid, 1);
+}
+
+/* See inferior.h.  */
+
+void
+interrupt_target_1 (bool all_threads)
+{
   if (non_stop)
-    set_stop_requested (ptid, 1);
+    {
+      if (all_threads)
+	{
+	  scoped_restore_current_thread restore_thread;
+
+	  for (inferior *inf : all_inferiors ())
+	    {
+	      switch_to_inferior_no_thread (inf);
+	      stop_current_target_threads_ns (minus_one_ptid);
+	    }
+	}
+      else
+	stop_current_target_threads_ns (inferior_ptid);
+    }
+  else
+    target_interrupt ();
 }
 
 /* interrupt [-a]
diff --git a/gdb/inferior-iter.h b/gdb/inferior-iter.h
index b1b595735b..0b2a3e55d8 100644
--- a/gdb/inferior-iter.h
+++ b/gdb/inferior-iter.h
@@ -36,18 +36,24 @@  public:
   typedef int difference_type;
 
   /* Create an iterator pointing at HEAD.  */
-  explicit all_inferiors_iterator (inferior *head)
-    : m_inf (head)
-  {}
+  explicit all_inferiors_iterator (process_stratum_target *proc_target,
+				   inferior *head)
+    : m_proc_target (proc_target)
+  {
+    /* Advance M_INF to the first inferior's position.  */
+    for (m_inf = head; m_inf != NULL; m_inf = m_inf->next)
+      if (m_inf_matches ())
+	return;
+  }
 
   /* Create a one-past-end iterator.  */
   all_inferiors_iterator ()
-    : m_inf (nullptr)
+    : m_proc_target (nullptr), m_inf (nullptr)
   {}
 
   all_inferiors_iterator &operator++ ()
   {
-    m_inf = m_inf->next;
+    advance ();
     return *this;
   }
 
@@ -58,6 +64,30 @@  public:
   { return m_inf != other.m_inf; }
 
 private:
+  /* Advance to next inferior, skipping filtered inferiors.  */
+  void advance ()
+  {
+    /* The loop below is written in the natural way as-if we'd always
+       start at the beginning of the inferior list.  This
+       fast-forwards the algorithm to the actual current position.  */
+    goto start;
+
+    while (m_inf != NULL)
+      {
+	if (m_inf_matches ())
+	  return;
+      start:
+	m_inf = m_inf->next;
+      }
+  }
+
+  bool m_inf_matches ()
+  {
+    return (m_proc_target == nullptr
+	    || m_proc_target == m_inf->process_target ());
+  }
+
+  process_stratum_target *m_proc_target;
   inferior *m_inf;
 };
 
@@ -80,10 +110,17 @@  using all_non_exited_inferiors_iterator
    inferiors with range-for.  */
 struct all_inferiors_range
 {
+  all_inferiors_range (process_stratum_target *proc_target = nullptr)
+    : m_filter_target (proc_target)
+  {}
+
   all_inferiors_iterator begin () const
-  { return all_inferiors_iterator (inferior_list); }
+  { return all_inferiors_iterator (m_filter_target, inferior_list); }
   all_inferiors_iterator end () const
   { return all_inferiors_iterator (); }
+
+private:
+  process_stratum_target *m_filter_target;
 };
 
 /* Iterate over all inferiors, safely.  */
@@ -97,10 +134,22 @@  using all_inferiors_safe_iterator
 
 struct all_inferiors_safe_range
 {
+  explicit all_inferiors_safe_range (process_stratum_target *filter_target)
+    : m_filter_target (filter_target)
+  {}
+
+  all_inferiors_safe_range ()
+    : m_filter_target (nullptr)
+  {}
+
   all_inferiors_safe_iterator begin () const
-  { return all_inferiors_safe_iterator (inferior_list); }
+  { return all_inferiors_safe_iterator (m_filter_target, inferior_list); }
   all_inferiors_safe_iterator end () const
   { return all_inferiors_safe_iterator (); }
+
+private:
+  /* The filter.  */
+  process_stratum_target *m_filter_target;
 };
 
 /* A range adapter that makes it possible to iterate over all
@@ -108,10 +157,22 @@  struct all_inferiors_safe_range
 
 struct all_non_exited_inferiors_range
 {
+  explicit all_non_exited_inferiors_range (process_stratum_target *filter_target)
+    : m_filter_target (filter_target)
+  {}
+
+  all_non_exited_inferiors_range ()
+    : m_filter_target (nullptr)
+  {}
+
   all_non_exited_inferiors_iterator begin () const
-  { return all_non_exited_inferiors_iterator (inferior_list); }
+  { return all_non_exited_inferiors_iterator (m_filter_target, inferior_list); }
   all_non_exited_inferiors_iterator end () const
   { return all_non_exited_inferiors_iterator (); }
+
+private:
+  /* The filter.  */
+  process_stratum_target *m_filter_target;
 };
 
 #endif /* !defined (INFERIOR_ITER_H) */
diff --git a/gdb/inferior.c b/gdb/inferior.c
index 97cecbe78b..6f4f396453 100644
--- a/gdb/inferior.c
+++ b/gdb/inferior.c
@@ -90,6 +90,8 @@  inferior::inferior (int pid_)
     registry_data ()
 {
   inferior_alloc_data (this);
+
+  m_target_stack.push (get_dummy_target ());
 }
 
 struct inferior *
@@ -275,14 +277,14 @@  find_inferior_id (int num)
 }
 
 struct inferior *
-find_inferior_pid (int pid)
+find_inferior_pid (process_stratum_target *targ, int pid)
 {
   /* Looking for inferior pid == 0 is always wrong, and indicative of
      a bug somewhere else.  There may be more than one with pid == 0,
      for instance.  */
   gdb_assert (pid != 0);
 
-  for (inferior *inf : all_inferiors ())
+  for (inferior *inf : all_inferiors (targ))
     if (inf->pid == pid)
       return inf;
 
@@ -292,9 +294,9 @@  find_inferior_pid (int pid)
 /* See inferior.h */
 
 struct inferior *
-find_inferior_ptid (ptid_t ptid)
+find_inferior_ptid (process_stratum_target *targ, ptid_t ptid)
 {
-  return find_inferior_pid (ptid.pid ());
+  return find_inferior_pid (targ, ptid.pid ());
 }
 
 /* See inferior.h.  */
@@ -339,11 +341,11 @@  have_inferiors (void)
    in the middle of a 'mourn' operation.  */
 
 int
-number_of_live_inferiors (void)
+number_of_live_inferiors (process_stratum_target *proc_target)
 {
   int num_inf = 0;
 
-  for (inferior *inf : all_non_exited_inferiors ())
+  for (inferior *inf : all_non_exited_inferiors (proc_target))
     if (inf->has_execution ())
       for (thread_info *tp ATTRIBUTE_UNUSED : inf->non_exited_threads ())
 	{
@@ -361,7 +363,7 @@  number_of_live_inferiors (void)
 int
 have_live_inferiors (void)
 {
-  return number_of_live_inferiors () > 0;
+  return number_of_live_inferiors (NULL) > 0;
 }
 
 /* Prune away any unused inferiors, and then prune away no longer used
@@ -695,7 +697,28 @@  add_inferior_with_spaces (void)
   return inf;
 }
 
-/* add-inferior [-copies N] [-exec FILENAME]  */
+/* Switch to inferior NEW_INF, a new inferior, and unless
+   NO_CONNECTION is true, push the process_stratum_target of ORG_INF
+   to NEW_INF.  */
+
+static void
+switch_to_inferior_and_push_target (inferior *new_inf,
+				    bool no_connection, inferior *org_inf)
+{
+  process_stratum_target *proc_target = org_inf->process_target ();
+
+  /* Switch over temporarily, while reading executable and
+     symbols.  */
+  switch_to_inferior_no_thread (new_inf);
+
+  /* Reuse the target for new inferior.  */
+  if (!no_connection && proc_target != NULL)
+    push_target (proc_target);
+
+  printf_filtered (_("Added inferior %d\n"), new_inf->num);
+}
+
+/* add-inferior [-copies N] [-exec FILENAME] [-no-connection] */
 
 static void
 add_inferior_command (const char *args, int from_tty)
@@ -703,6 +726,7 @@  add_inferior_command (const char *args, int from_tty)
   int i, copies = 1;
   gdb::unique_xmalloc_ptr<char> exec;
   symfile_add_flags add_flags = 0;
+  bool no_connection = false;
 
   if (from_tty)
     add_flags |= SYMFILE_VERBOSE;
@@ -722,6 +746,8 @@  add_inferior_command (const char *args, int from_tty)
 		    error (_("No argument to -copies"));
 		  copies = parse_and_eval_long (*argv);
 		}
+	      else if (strcmp (*argv, "-no-connection") == 0)
+		no_connection = true;
 	      else if (strcmp (*argv, "-exec") == 0)
 		{
 		  ++argv;
@@ -735,32 +761,32 @@  add_inferior_command (const char *args, int from_tty)
 	}
     }
 
+  inferior *orginf = current_inferior ();
+
   scoped_restore_current_pspace_and_thread restore_pspace_thread;
 
   for (i = 0; i < copies; ++i)
     {
-      struct inferior *inf = add_inferior_with_spaces ();
+      inferior *inf = add_inferior_with_spaces ();
 
-      printf_filtered (_("Added inferior %d\n"), inf->num);
+      switch_to_inferior_and_push_target (inf, no_connection, orginf);
 
       if (exec != NULL)
 	{
-	  /* Switch over temporarily, while reading executable and
-	     symbols.  */
-	  switch_to_inferior_no_thread (inf);
 	  exec_file_attach (exec.get (), from_tty);
 	  symbol_file_add_main (exec.get (), add_flags);
 	}
     }
 }
 
-/* clone-inferior [-copies N] [ID] */
+/* clone-inferior [-copies N] [ID] [-no-connection] */
 
 static void
 clone_inferior_command (const char *args, int from_tty)
 {
   int i, copies = 1;
   struct inferior *orginf = NULL;
+  bool no_connection = false;
 
   if (args)
     {
@@ -781,6 +807,8 @@  clone_inferior_command (const char *args, int from_tty)
 		  if (copies < 0)
 		    error (_("Invalid copies number"));
 		}
+	      else if (strcmp (*argv, "-no-connection") == 0)
+		no_connection = true;
 	    }
 	  else
 	    {
@@ -826,15 +854,13 @@  clone_inferior_command (const char *args, int from_tty)
       inf->aspace = pspace->aspace;
       inf->gdbarch = orginf->gdbarch;
 
+      switch_to_inferior_and_push_target (inf, no_connection, orginf);
+
       /* If the original inferior had a user specified target
 	 description, make the clone use it too.  */
       if (target_desc_info_from_user_p (inf->tdesc_info))
 	copy_inferior_target_desc_info (inf, orginf);
 
-      printf_filtered (_("Added inferior %d.\n"), inf->num);
-
-      set_current_inferior (inf);
-      switch_to_no_thread ();
       clone_program_space (pspace, orginf->pspace);
     }
 }
@@ -895,10 +921,13 @@  By default all inferiors are displayed."));
 
   c = add_com ("add-inferior", no_class, add_inferior_command, _("\
 Add a new inferior.\n\
-Usage: add-inferior [-copies N] [-exec FILENAME]\n\
+Usage: add-inferior [-copies N] [-exec FILENAME] [-no-connection]\n\
 N is the optional number of inferiors to add, default is 1.\n\
 FILENAME is the file name of the executable to use\n\
-as main program."));
+as main program.\n\
+By default, the new inferior inherits the current inferior's connection.\n\
+If -no-connection is specified, the new inferior begins with\n\
+no target connection yet."));
   set_cmd_completer (c, filename_completer);
 
   add_com ("remove-inferiors", no_class, remove_inferior_command, _("\
@@ -907,11 +936,14 @@  Usage: remove-inferiors ID..."));
 
   add_com ("clone-inferior", no_class, clone_inferior_command, _("\
 Clone inferior ID.\n\
-Usage: clone-inferior [-copies N] [ID]\n\
-Add N copies of inferior ID.  The new inferior has the same\n\
+Usage: clone-inferior [-copies N] [-no-connection] [ID]\n\
+Add N copies of inferior ID.  The new inferiors have the same\n\
 executable loaded as the copied inferior.  If -copies is not specified,\n\
 adds 1 copy.  If ID is not specified, it is the current inferior\n\
-that is cloned."));
+that is cloned.\n\
+By default, the new inferiors inherit the copied inferior's connection.\n\
+If -no-connection is specified, the new inferiors begin with\n\
+no target connection yet."));
 
   add_cmd ("inferiors", class_run, detach_inferior_command, _("\
 Detach from inferior ID (or list of IDS).\n\
diff --git a/gdb/inferior.h b/gdb/inferior.h
index 967d4fa8b7..3bcb6d6c60 100644
--- a/gdb/inferior.h
+++ b/gdb/inferior.h
@@ -56,6 +56,8 @@  struct thread_info;
 #include "gdbsupport/common-inferior.h"
 #include "gdbthread.h"
 
+#include "process-stratum-target.h"
+
 struct infcall_suspend_state;
 struct infcall_control_state;
 
@@ -208,7 +210,7 @@  extern void registers_info (const char *, int);
 
 extern void continue_1 (int all_threads);
 
-extern void interrupt_target_1 (int all_threads);
+extern void interrupt_target_1 (bool all_threads);
 
 using delete_longjmp_breakpoint_cleanup
   = FORWARD_SCOPE_EXIT (delete_longjmp_breakpoint);
@@ -364,6 +366,35 @@  public:
   /* Returns true if we can delete this inferior.  */
   bool deletable () const { return refcount () == 0; }
 
+  /* Push T in this inferior's target stack.  */
+  void push_target (struct target_ops *t)
+  { m_target_stack.push (t); }
+
+  /* Unpush T from this inferior's target stack.  */
+  int unpush_target (struct target_ops *t)
+  { return m_target_stack.unpush (t); }
+
+  /* Returns true if T is pushed in this inferior's target stack.  */
+  bool target_is_pushed (target_ops *t)
+  { return m_target_stack.is_pushed (t); }
+
+  /* Find the target beneath T in this inferior's target stack.  */
+  target_ops *find_target_beneath (const target_ops *t)
+  { return m_target_stack.find_beneath (t); }
+
+  /* Return the target at the top of this inferior's target stack.  */
+  target_ops *top_target ()
+  { return m_target_stack.top (); }
+
+  /* Return the target at process_stratum level in this inferior's
+     target stack.  */
+  struct process_stratum_target *process_target ()
+  { return (process_stratum_target *) m_target_stack.at (process_stratum); }
+
+  /* Return the target at STRATUM in this inferior's target stack.  */
+  target_ops *target_at (enum strata stratum)
+  { return m_target_stack.at (stratum); }
+
   bool has_execution ()
   { return target_has_execution_1 (this); }
 
@@ -528,6 +559,10 @@  public:
 
   /* Per inferior data-pointers required by other GDB modules.  */
   REGISTRY_FIELDS;
+
+private:
+  /* The inferior's target stack.  */
+  target_stack m_target_stack;
 };
 
 /* Keep a registry of per-inferior data-pointers required by other GDB
@@ -558,14 +593,14 @@  extern void exit_inferior_num_silent (int num);
 
 extern void inferior_appeared (struct inferior *inf, int pid);
 
-/* Get rid of all inferiors.  */
-extern void discard_all_inferiors (void);
+/* Search function to lookup an inferior of TARG by target 'pid'.  */
+extern struct inferior *find_inferior_pid (process_stratum_target *targ,
+					   int pid);
 
-/* Search function to lookup an inferior by target 'pid'.  */
-extern struct inferior *find_inferior_pid (int pid);
-
-/* Search function to lookup an inferior whose pid is equal to 'ptid.pid'. */
-extern struct inferior *find_inferior_ptid (ptid_t ptid);
+/* Search function to lookup an inferior of TARG whose pid is equal to
+   'ptid.pid'. */
+extern struct inferior *find_inferior_ptid (process_stratum_target *targ,
+					    ptid_t ptid);
 
 /* Search function to lookup an inferior by GDB 'num'.  */
 extern struct inferior *find_inferior_id (int num);
@@ -592,8 +627,9 @@  extern struct inferior *iterate_over_inferiors (int (*) (struct inferior *,
 /* Returns true if the inferior list is not empty.  */
 extern int have_inferiors (void);
 
-/* Returns the number of live inferiors (real live processes).  */
-extern int number_of_live_inferiors (void);
+/* Returns the number of live inferiors running on PROC_TARGET (real
+   live processes with execution).  */
+extern int number_of_live_inferiors (process_stratum_target *proc_target);
 
 /* Returns true if there are any live inferiors in the inferior list
    (not cores, not executables, real live processes).  */
@@ -650,18 +686,18 @@  all_inferiors_safe ()
 */
 
 inline all_inferiors_range
-all_inferiors ()
+all_inferiors (process_stratum_target *proc_target = nullptr)
 {
-  return {};
+  return all_inferiors_range (proc_target);
 }
 
 /* Return a range that can be used to walk over all inferiors with PID
    not zero, with range-for.  */
 
 inline all_non_exited_inferiors_range
-all_non_exited_inferiors ()
+all_non_exited_inferiors (process_stratum_target *proc_target = nullptr)
 {
-  return {};
+  return all_non_exited_inferiors_range (proc_target);
 }
 
 /* Prune away automatically added inferiors that aren't required
diff --git a/gdb/infrun.c b/gdb/infrun.c
index d27b47c6a8..33088a11e7 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -69,6 +69,7 @@ 
 #include "arch-utils.h"
 #include "gdbsupport/scope-exit.h"
 #include "gdbsupport/forward-scope-exit.h"
+#include "gdb_select.h"
 
 /* Prototypes for local functions */
 
@@ -94,6 +95,8 @@  static int maybe_software_singlestep (struct gdbarch *gdbarch, CORE_ADDR pc);
 
 static void resume (gdb_signal sig);
 
+static void wait_for_inferior (inferior *inf);
+
 /* Asynchronous signal handler registered as event loop source for
    when we have pending events ready to be passed to the core.  */
 static struct async_event_handler *infrun_async_inferior_event_token;
@@ -378,9 +381,10 @@  show_stop_on_solib_events (struct ui_file *file, int from_tty,
 
 static int stop_print_frame;
 
-/* This is a cached copy of the pid/waitstatus of the last event
-   returned by target_wait()/deprecated_target_wait_hook().  This
-   information is returned by get_last_target_status().  */
+/* This is a cached copy of the target/ptid/waitstatus of the last
+   event returned by target_wait()/deprecated_target_wait_hook().
+   This information is returned by get_last_target_status().  */
+static process_stratum_target *target_last_proc_target;
 static ptid_t target_last_wait_ptid;
 static struct target_waitstatus target_last_waitstatus;
 
@@ -487,10 +491,12 @@  holding the child stopped.  Try \"set detach-on-fork\" or \
 
 	  scoped_restore_current_pspace_and_thread restore_pspace_thread;
 
-	  inferior_ptid = child_ptid;
-	  add_thread_silent (inferior_ptid);
 	  set_current_inferior (child_inf);
+	  switch_to_no_thread ();
 	  child_inf->symfile_flags = SYMFILE_NO_READ;
+	  push_target (parent_inf->process_target ());
+	  add_thread_silent (child_inf->process_target (), child_ptid);
+	  inferior_ptid = child_ptid;
 
 	  /* If this is a vfork child, then the address-space is
 	     shared with the parent.  */
@@ -499,6 +505,8 @@  holding the child stopped.  Try \"set detach-on-fork\" or \
 	      child_inf->pspace = parent_inf->pspace;
 	      child_inf->aspace = parent_inf->aspace;
 
+	      exec_on_vfork ();
+
 	      /* The parent will be frozen until the child is done
 		 with the shared region.  Keep track of the
 		 parent.  */
@@ -574,52 +582,64 @@  holding the child stopped.  Try \"set detach-on-fork\" or \
 
       parent_pspace = parent_inf->pspace;
 
-      /* If we're vforking, we want to hold on to the parent until the
-	 child exits or execs.  At child exec or exit time we can
-	 remove the old breakpoints from the parent and detach or
-	 resume debugging it.  Otherwise, detach the parent now; we'll
-	 want to reuse it's program/address spaces, but we can't set
-	 them to the child before removing breakpoints from the
-	 parent, otherwise, the breakpoints module could decide to
-	 remove breakpoints from the wrong process (since they'd be
-	 assigned to the same address space).  */
+      process_stratum_target *target = parent_inf->process_target ();
 
-      if (has_vforked)
-	{
-	  gdb_assert (child_inf->vfork_parent == NULL);
-	  gdb_assert (parent_inf->vfork_child == NULL);
-	  child_inf->vfork_parent = parent_inf;
-	  child_inf->pending_detach = 0;
-	  parent_inf->vfork_child = child_inf;
-	  parent_inf->pending_detach = detach_fork;
-	  parent_inf->waiting_for_vfork_done = 0;
-	}
-      else if (detach_fork)
-	{
-	  if (print_inferior_events)
-	    {
-	      /* Ensure that we have a process ptid.  */
-	      ptid_t process_ptid = ptid_t (parent_ptid.pid ());
+      {
+	/* Hold a strong reference to the target while (maybe)
+	   detaching the parent.  Otherwise detaching could close the
+	   target.  */
+	auto target_ref = target_ops_ref::new_reference (target);
+
+	/* If we're vforking, we want to hold on to the parent until
+	   the child exits or execs.  At child exec or exit time we
+	   can remove the old breakpoints from the parent and detach
+	   or resume debugging it.  Otherwise, detach the parent now;
+	   we'll want to reuse it's program/address spaces, but we
+	   can't set them to the child before removing breakpoints
+	   from the parent, otherwise, the breakpoints module could
+	   decide to remove breakpoints from the wrong process (since
+	   they'd be assigned to the same address space).  */
+
+	if (has_vforked)
+	  {
+	    gdb_assert (child_inf->vfork_parent == NULL);
+	    gdb_assert (parent_inf->vfork_child == NULL);
+	    child_inf->vfork_parent = parent_inf;
+	    child_inf->pending_detach = 0;
+	    parent_inf->vfork_child = child_inf;
+	    parent_inf->pending_detach = detach_fork;
+	    parent_inf->waiting_for_vfork_done = 0;
+	  }
+	else if (detach_fork)
+	  {
+	    if (print_inferior_events)
+	      {
+		/* Ensure that we have a process ptid.  */
+		ptid_t process_ptid = ptid_t (parent_ptid.pid ());
+
+		target_terminal::ours_for_output ();
+		fprintf_filtered (gdb_stdlog,
+				  _("[Detaching after fork from "
+				    "parent %s]\n"),
+				  target_pid_to_str (process_ptid).c_str ());
+	      }
 
-	      target_terminal::ours_for_output ();
-	      fprintf_filtered (gdb_stdlog,
-				_("[Detaching after fork from "
-				  "parent %s]\n"),
-				target_pid_to_str (process_ptid).c_str ());
-	    }
+	    target_detach (parent_inf, 0);
+	    parent_inf = NULL;
+	  }
 
-	  target_detach (parent_inf, 0);
-	}
+	/* Note that the detach above makes PARENT_INF dangling.  */
 
-      /* Note that the detach above makes PARENT_INF dangling.  */
+	/* Add the child thread to the appropriate lists, and switch
+	   to this new thread, before cloning the program space, and
+	   informing the solib layer about this new process.  */
 
-      /* Add the child thread to the appropriate lists, and switch to
-	 this new thread, before cloning the program space, and
-	 informing the solib layer about this new process.  */
+	set_current_inferior (child_inf);
+	push_target (target);
+      }
 
+      add_thread_silent (target, child_ptid);
       inferior_ptid = child_ptid;
-      add_thread_silent (inferior_ptid);
-      set_current_inferior (child_inf);
 
       /* If this is a vfork child, then the address-space is shared
 	 with the parent.  If we detached from the parent, then we can
@@ -628,6 +648,8 @@  holding the child stopped.  Try \"set detach-on-fork\" or \
 	{
 	  child_inf->pspace = parent_pspace;
 	  child_inf->aspace = child_inf->pspace->aspace;
+
+	  exec_on_vfork ();
 	}
       else
 	{
@@ -674,11 +696,12 @@  follow_fork (void)
 
   if (!non_stop)
     {
+      process_stratum_target *wait_target;
       ptid_t wait_ptid;
       struct target_waitstatus wait_status;
 
       /* Get the last target status returned by target_wait().  */
-      get_last_target_status (&wait_ptid, &wait_status);
+      get_last_target_status (&wait_target, &wait_ptid, &wait_status);
 
       /* If not stopped at a fork event, then there's nothing else to
 	 do.  */
@@ -689,14 +712,14 @@  follow_fork (void)
       /* Check if we switched over from WAIT_PTID, since the event was
 	 reported.  */
       if (wait_ptid != minus_one_ptid
-	  && inferior_ptid != wait_ptid)
+	  && (current_inferior ()->process_target () != wait_target
+	      || inferior_ptid != wait_ptid))
 	{
 	  /* We did.  Switch back to WAIT_PTID thread, to tell the
 	     target to follow it (in either direction).  We'll
 	     afterwards refuse to resume, and inform the user what
 	     happened.  */
-	  thread_info *wait_thread
-	    = find_thread_ptid (wait_ptid);
+	  thread_info *wait_thread = find_thread_ptid (wait_target, wait_ptid);
 	  switch_to_thread (wait_thread);
 	  should_resume = 0;
 	}
@@ -742,6 +765,7 @@  follow_fork (void)
 	parent = inferior_ptid;
 	child = tp->pending_follow.value.related_pid;
 
+	process_stratum_target *parent_targ = tp->inf->process_target ();
 	/* Set up inferior(s) as specified by the caller, and tell the
 	   target to do whatever is necessary to follow either parent
 	   or child.  */
@@ -757,7 +781,7 @@  follow_fork (void)
 	       or another.  The previous selected thread may be gone
 	       from the lists by now, but if it is still around, need
 	       to clear the pending follow request.  */
-	    tp = find_thread_ptid (parent);
+	    tp = find_thread_ptid (parent_targ, parent);
 	    if (tp)
 	      tp->pending_follow.kind = TARGET_WAITKIND_SPURIOUS;
 
@@ -768,7 +792,7 @@  follow_fork (void)
 	    /* If we followed the child, switch to it...  */
 	    if (follow_child)
 	      {
-		thread_info *child_thr = find_thread_ptid (child);
+		thread_info *child_thr = find_thread_ptid (parent_targ, child);
 		switch_to_thread (child_thr);
 
 		/* ... and preserve the stepping state, in case the
@@ -1197,9 +1221,11 @@  follow_exec (ptid_t ptid, const char *exec_file_target)
       inf->pid = pid;
       target_follow_exec (inf, exec_file_target);
 
-      set_current_inferior (inf);
-      set_current_program_space (inf->pspace);
-      add_thread (ptid);
+      inferior *org_inferior = current_inferior ();
+      switch_to_inferior_no_thread (inf);
+      push_target (org_inferior->process_target ());
+      thread_info *thr = add_thread (inf->process_target (), ptid);
+      switch_to_thread (thr);
     }
   else
     {
@@ -1893,6 +1919,7 @@  displaced_step_fixup (thread_info *event_thread, enum gdb_signal signal)
    discarded between events.  */
 struct execution_control_state
 {
+  process_stratum_target *target;
   ptid_t ptid;
   /* The thread that got the event, if this was a thread event; NULL
      otherwise.  */
@@ -2149,6 +2176,16 @@  user_visible_resume_ptid (int step)
   return resume_ptid;
 }
 
+/* See infrun.h.  */
+
+process_stratum_target *
+user_visible_resume_target (ptid_t resume_ptid)
+{
+  return (resume_ptid == minus_one_ptid
+	  ? NULL
+	  : current_inferior ()->process_target ());
+}
+
 /* Return a ptid representing the set of threads that we will resume,
    in the perspective of the target, assuming run control handling
    does not require leaving some threads stopped (e.g., stepping past
@@ -2256,6 +2293,7 @@  resume_1 (enum gdb_signal sig)
 			      currently_stepping (tp));
 	}
 
+      tp->inf->process_target ()->threads_executing = true;
       tp->resumed = 1;
 
       /* FIXME: What should we do if we are supposed to resume this
@@ -2741,10 +2779,12 @@  clear_proceed_status (int step)
   if (!non_stop && inferior_ptid != null_ptid)
     {
       ptid_t resume_ptid = user_visible_resume_ptid (step);
+      process_stratum_target *resume_target
+	= user_visible_resume_target (resume_ptid);
 
       /* In all-stop mode, delete the per-thread status of all threads
 	 we're about to resume, implicitly and explicitly.  */
-      for (thread_info *tp : all_non_exited_threads (resume_ptid))
+      for (thread_info *tp : all_non_exited_threads (resume_target, resume_ptid))
 	clear_proceed_status_thread (tp);
     }
 
@@ -2821,6 +2861,21 @@  schedlock_applies (struct thread_info *tp)
 					    execution_direction)));
 }
 
+/* Calls target_commit_resume on all targets.  */
+
+static void
+commit_resume_all_targets ()
+{
+  scoped_restore_current_thread restore_thread;
+
+  for (inferior *inf : all_non_exited_inferiors ())
+    if (inf->has_execution ())
+      {
+	switch_to_inferior_no_thread (inf);
+	target_commit_resume ();
+      }
+}
+
 /* Basic routine for continuing the program in various fashions.
 
    ADDR is the address to resume at, or -1 for resume where stopped.
@@ -2835,7 +2890,6 @@  proceed (CORE_ADDR addr, enum gdb_signal siggnal)
   struct regcache *regcache;
   struct gdbarch *gdbarch;
   CORE_ADDR pc;
-  ptid_t resume_ptid;
   struct execution_control_state ecss;
   struct execution_control_state *ecs = &ecss;
   int started;
@@ -2867,6 +2921,11 @@  proceed (CORE_ADDR addr, enum gdb_signal siggnal)
 
   gdb_assert (!thread_is_in_step_over_chain (cur_thr));
 
+  ptid_t resume_ptid
+    = user_visible_resume_ptid (cur_thr->control.stepping_command);
+  process_stratum_target *resume_target
+    = user_visible_resume_target (resume_ptid);
+
   if (addr == (CORE_ADDR) -1)
     {
       if (pc == cur_thr->suspend.stop_pc
@@ -2896,12 +2955,10 @@  proceed (CORE_ADDR addr, enum gdb_signal siggnal)
   if (siggnal != GDB_SIGNAL_DEFAULT)
     cur_thr->suspend.stop_signal = siggnal;
 
-  resume_ptid = user_visible_resume_ptid (cur_thr->control.stepping_command);
-
   /* If an exception is thrown from this point on, make sure to
      propagate GDB's knowledge of the executing state to the
      frontend/user running state.  */
-  scoped_finish_thread_state finish_state (resume_ptid);
+  scoped_finish_thread_state finish_state (resume_target, resume_ptid);
 
   /* Even if RESUME_PTID is a wildcard, and we end up resuming fewer
      threads (e.g., we might need to set threads stepping over
@@ -2910,7 +2967,7 @@  proceed (CORE_ADDR addr, enum gdb_signal siggnal)
      inferior function, as in that case we pretend the inferior
      doesn't run at all.  */
   if (!cur_thr->control.in_infcall)
-   set_running (resume_ptid, 1);
+    set_running (resume_target, resume_ptid, 1);
 
   if (debug_infrun)
     fprintf_unfiltered (gdb_stdlog,
@@ -2945,7 +3002,8 @@  proceed (CORE_ADDR addr, enum gdb_signal siggnal)
      threads.  */
   if (!non_stop && !schedlock_applies (cur_thr))
     {
-      for (thread_info *tp : all_non_exited_threads (resume_ptid))
+      for (thread_info *tp : all_non_exited_threads (resume_target,
+						     resume_ptid))
 	{
 	  switch_to_thread_no_regs (tp);
 
@@ -3002,9 +3060,20 @@  proceed (CORE_ADDR addr, enum gdb_signal siggnal)
       {
 	/* In all-stop, but the target is always in non-stop mode.
 	   Start all other threads that are implicitly resumed too.  */
-      for (thread_info *tp : all_non_exited_threads (resume_ptid))
-        {
-	  switch_to_thread_no_regs (tp);
+	for (thread_info *tp : all_non_exited_threads (resume_target,
+						       resume_ptid))
+	  {
+	    switch_to_thread_no_regs (tp);
+
+	  if (!tp->inf->has_execution ())
+	    {
+	      if (debug_infrun)
+		fprintf_unfiltered (gdb_stdlog,
+				    "infrun: proceed: [%s] target has "
+				    "no execution\n",
+				    target_pid_to_str (tp->ptid).c_str ());
+	      continue;
+	    }
 
 	  if (tp->resumed)
 	    {
@@ -3048,7 +3117,7 @@  proceed (CORE_ADDR addr, enum gdb_signal siggnal)
       }
   }
 
-  target_commit_resume ();
+  commit_resume_all_targets ();
 
   finish_state.release ();
 
@@ -3070,10 +3139,8 @@  proceed (CORE_ADDR addr, enum gdb_signal siggnal)
 void
 start_remote (int from_tty)
 {
-  struct inferior *inferior;
-
-  inferior = current_inferior ();
-  inferior->control.stop_soon = STOP_QUIETLY_REMOTE;
+  inferior *inf = current_inferior ();
+  inf->control.stop_soon = STOP_QUIETLY_REMOTE;
 
   /* Always go on waiting for the target, regardless of the mode.  */
   /* FIXME: cagney/1999-09-23: At present it isn't possible to
@@ -3089,7 +3156,7 @@  start_remote (int from_tty)
      target_open() return to the caller an indication that the target
      is currently running and GDB state should be set to the same as
      for an async run.  */
-  wait_for_inferior ();
+  wait_for_inferior (inf);
 
   /* Now that the inferior has stopped, do any bookkeeping like
      loading shared libraries.  We want to do this before normal_stop,
@@ -3140,11 +3207,13 @@  static int switch_back_to_stepped_thread (struct execution_control_state *ecs);
 static void
 infrun_thread_stop_requested (ptid_t ptid)
 {
+  process_stratum_target *curr_target = current_inferior ()->process_target ();
+
   /* PTID was requested to stop.  If the thread was already stopped,
      but the user/frontend doesn't know about that yet (e.g., the
      thread had been temporarily paused for some step-over), set up
      for reporting the stop now.  */
-  for (thread_info *tp : all_threads (ptid))
+  for (thread_info *tp : all_threads (curr_target, ptid))
     {
       if (tp->state != THREAD_RUNNING)
 	continue;
@@ -3170,7 +3239,7 @@  infrun_thread_stop_requested (ptid_t ptid)
 
       /* Clear the inline-frame state, since we're re-processing the
 	 stop.  */
-      clear_inline_frame_state (tp->ptid);
+      clear_inline_frame_state (tp);
 
       /* If this thread was paused because some other thread was
 	 doing an inline-step over, let that finish first.  Once
@@ -3189,7 +3258,8 @@  infrun_thread_stop_requested (ptid_t ptid)
 static void
 infrun_thread_thread_exit (struct thread_info *tp, int silent)
 {
-  if (target_last_wait_ptid == tp->ptid)
+  if (target_last_proc_target == tp->inf->process_target ()
+      && target_last_wait_ptid == tp->ptid)
     nullify_last_target_wait_ptid ();
 }
 
@@ -3285,19 +3355,20 @@  print_target_wait_results (ptid_t waiton_ptid, ptid_t result_ptid,
    had events.  */
 
 static struct thread_info *
-random_pending_event_thread (ptid_t waiton_ptid)
+random_pending_event_thread (inferior *inf, ptid_t waiton_ptid)
 {
   int num_events = 0;
 
-  auto has_event = [] (thread_info *tp)
+  auto has_event = [&] (thread_info *tp)
     {
-      return (tp->resumed
+      return (tp->ptid.matches (waiton_ptid)
+	      && tp->resumed
 	      && tp->suspend.waitstatus_pending_p);
     };
 
   /* First see how many events we have.  Count only resumed threads
      that have an event pending.  */
-  for (thread_info *tp : all_non_exited_threads (waiton_ptid))
+  for (thread_info *tp : inf->non_exited_threads ())
     if (has_event (tp))
       num_events++;
 
@@ -3314,7 +3385,7 @@  random_pending_event_thread (ptid_t waiton_ptid)
 			num_events, random_selector);
 
   /* Select the Nth thread that has had an event.  */
-  for (thread_info *tp : all_non_exited_threads (waiton_ptid))
+  for (thread_info *tp : inf->non_exited_threads ())
     if (has_event (tp))
       if (random_selector-- == 0)
 	return tp;
@@ -3324,10 +3395,12 @@  random_pending_event_thread (ptid_t waiton_ptid)
 
 /* Wrapper for target_wait that first checks whether threads have
    pending statuses to report before actually asking the target for
-   more events.  */
+   more events.  INF is the inferior we're using to call target_wait
+   on.  */
 
 static ptid_t
-do_target_wait (ptid_t ptid, struct target_waitstatus *status, int options)
+do_target_wait_1 (inferior *inf, ptid_t ptid,
+		  target_waitstatus *status, int options)
 {
   ptid_t event_ptid;
   struct thread_info *tp;
@@ -3336,7 +3409,7 @@  do_target_wait (ptid_t ptid, struct target_waitstatus *status, int options)
      pending.  */
   if (ptid == minus_one_ptid || ptid.is_pid ())
     {
-      tp = random_pending_event_thread (ptid);
+      tp = random_pending_event_thread (inf, ptid);
     }
   else
     {
@@ -3346,7 +3419,7 @@  do_target_wait (ptid_t ptid, struct target_waitstatus *status, int options)
 			    target_pid_to_str (ptid).c_str ());
 
       /* We have a specific thread to check.  */
-      tp = find_thread_ptid (ptid);
+      tp = find_thread_ptid (inf, ptid);
       gdb_assert (tp != NULL);
       if (!tp->suspend.waitstatus_pending_p)
 	tp = NULL;
@@ -3453,6 +3526,109 @@  do_target_wait (ptid_t ptid, struct target_waitstatus *status, int options)
   return event_ptid;
 }
 
+/* Returns true if INF has any resumed thread with a status
+   pending.  */
+
+static bool
+threads_are_resumed_pending_p (inferior *inf)
+{
+  for (thread_info *tp : inf->non_exited_threads ())
+    if (tp->resumed
+	&& tp->suspend.waitstatus_pending_p)
+      return true;
+
+  return false;
+}
+
+/* Wrapper for target_wait that first checks whether threads have
+   pending statuses to report before actually asking the target for
+   more events. Polls for events from all inferiors/targets.  */
+
+static bool
+do_target_wait (ptid_t wait_ptid, execution_control_state *ecs, int options)
+{
+  int num_inferiors = 0;
+  int random_selector;
+
+  /* For fairness, we pick the first inferior/target to poll at
+     random, and then continue polling the rest of the inferior list
+     starting from that one in a circular fashion until the whole list
+     is polled once.  */
+
+  auto inferior_matches = [&wait_ptid] (inferior *inf)
+    {
+      return (inf->process_target () != NULL
+	      && (threads_are_executing (inf->process_target ())
+		  || threads_are_resumed_pending_p (inf))
+	      && ptid_t (inf->pid).matches (wait_ptid));
+    };
+
+  /* First see how many resumed inferiors we have.  */
+  for (inferior *inf : all_inferiors ())
+    if (inferior_matches (inf))
+      num_inferiors++;
+
+  if (num_inferiors == 0)
+    {
+      ecs->ws.kind = TARGET_WAITKIND_IGNORE;
+      return false;
+    }
+
+  /* Now randomly pick an inferior out of those that were resumed.  */
+  random_selector = (int)
+    ((num_inferiors * (double) rand ()) / (RAND_MAX + 1.0));
+
+  if (debug_infrun && num_inferiors > 1)
+    fprintf_unfiltered (gdb_stdlog,
+			"infrun: Found %d inferiors, starting at #%d\n",
+			num_inferiors, random_selector);
+
+  /* Select the Nth inferior that was resumed.  */
+
+  inferior *selected = nullptr;
+
+  for (inferior *inf : all_inferiors ())
+    if (inferior_matches (inf))
+      if (random_selector-- == 0)
+	{
+	  selected = inf;
+	  break;
+	}
+
+  /* Now poll for events out of each of the resumed inferior's
+     targets, starting from the selected one.  */
+
+  auto do_wait = [&] (inferior *inf)
+  {
+    switch_to_inferior_no_thread (inf);
+
+    ecs->ptid = do_target_wait_1 (inf, wait_ptid, &ecs->ws, options);
+    ecs->target = inf->process_target ();
+    return (ecs->ws.kind != TARGET_WAITKIND_IGNORE);
+  };
+
+  /* Needed in all-stop+target-non-stop mode, because we end up here
+     spuriously after the target is all stopped and we've already
+     reported the stop to the user, polling for events.  */
+  scoped_restore_current_thread restore_thread;
+
+  int inf_num = selected->num;
+  for (inferior *inf = selected; inf != NULL; inf = inf->next)
+    if (inferior_matches (inf))
+      if (do_wait (inf))
+	return true;
+
+  for (inferior *inf = inferior_list;
+       inf != NULL && inf->num < inf_num;
+       inf = inf->next)
+    if (inferior_matches (inf))
+      if (do_wait (inf))
+	return true;
+
+  ecs->ws.kind = TARGET_WAITKIND_IGNORE;
+  return false;
+}
+
 /* Prepare and stabilize the inferior for detaching it.  E.g.,
    detaching while a thread is displaced stepping is a recipe for
    crashing it, as nothing would readjust the PC out of the scratch
@@ -3492,7 +3668,7 @@  prepare_for_detach (void)
 	 don't get any event.  */
       target_dcache_invalidate ();
 
-      ecs->ptid = do_target_wait (pid_ptid, &ecs->ws, 0);
+      do_target_wait (pid_ptid, ecs, 0);
 
       if (debug_infrun)
 	print_target_wait_results (pid_ptid, ecs->ptid, &ecs->ws);
@@ -3500,7 +3676,8 @@  prepare_for_detach (void)
       /* If an error happens while handling the event, propagate GDB's
 	 knowledge of the executing state to the frontend/user running
 	 state.  */
-      scoped_finish_thread_state finish_state (minus_one_ptid);
+      scoped_finish_thread_state finish_state (inf->process_target (),
+					       minus_one_ptid);
 
       /* Now figure out what to do with the result of the result.  */
       handle_inferior_event (ecs);
@@ -3528,8 +3705,8 @@  prepare_for_detach (void)
    When this function actually returns it means the inferior
    should be left stopped and GDB should read more commands.  */
 
-void
-wait_for_inferior (void)
+static void
+wait_for_inferior (inferior *inf)
 {
   if (debug_infrun)
     fprintf_unfiltered
@@ -3540,13 +3717,13 @@  wait_for_inferior (void)
   /* If an error happens while handling the event, propagate GDB's
      knowledge of the executing state to the frontend/user running
      state.  */
-  scoped_finish_thread_state finish_state (minus_one_ptid);
+  scoped_finish_thread_state finish_state
+    (inf->process_target (), minus_one_ptid);
 
   while (1)
     {
       struct execution_control_state ecss;
       struct execution_control_state *ecs = &ecss;
-      ptid_t waiton_ptid = minus_one_ptid;
 
       memset (ecs, 0, sizeof (*ecs));
 
@@ -3558,10 +3735,11 @@  wait_for_inferior (void)
 	 don't get any event.  */
       target_dcache_invalidate ();
 
-      ecs->ptid = do_target_wait (waiton_ptid, &ecs->ws, 0);
+      ecs->ptid = do_target_wait_1 (inf, minus_one_ptid, &ecs->ws, 0);
+      ecs->target = inf->process_target ();
 
       if (debug_infrun)
-	print_target_wait_results (waiton_ptid, ecs->ptid, &ecs->ws);
+	print_target_wait_results (minus_one_ptid, ecs->ptid, &ecs->ws);
 
       /* Now figure out what to do with the result of the result.  */
       handle_inferior_event (ecs);
@@ -3687,7 +3865,6 @@  fetch_inferior_event (void *client_data)
   struct execution_control_state ecss;
   struct execution_control_state *ecs = &ecss;
   int cmd_done = 0;
-  ptid_t waiton_ptid = minus_one_ptid;
 
   memset (ecs, 0, sizeof (*ecs));
 
@@ -3728,17 +3905,28 @@  fetch_inferior_event (void *client_data)
       = make_scoped_restore (&execution_direction,
 			     target_execution_direction ());
 
-    ecs->ptid = do_target_wait (waiton_ptid, &ecs->ws,
-				target_can_async_p () ? TARGET_WNOHANG : 0);
+    if (!do_target_wait (minus_one_ptid, ecs, TARGET_WNOHANG))
+      return;
+
+    gdb_assert (ecs->ws.kind != TARGET_WAITKIND_IGNORE);
+
+    /* Switch to the target that generated the event, so we can do
+       target calls.  Any inferior bound to the target will do, so we
+       just switch to the first we find.  */
+    for (inferior *inf : all_inferiors (ecs->target))
+      {
+	switch_to_inferior_no_thread (inf);
+	break;
+      }
 
     if (debug_infrun)
-      print_target_wait_results (waiton_ptid, ecs->ptid, &ecs->ws);
+      print_target_wait_results (minus_one_ptid, ecs->ptid, &ecs->ws);
 
     /* If an error happens while handling the event, propagate GDB's
        knowledge of the executing state to the frontend/user running
        state.  */
     ptid_t finish_ptid = !target_is_non_stop_p () ? minus_one_ptid : ecs->ptid;
-    scoped_finish_thread_state finish_state (finish_ptid);
+    scoped_finish_thread_state finish_state (ecs->target, finish_ptid);
 
     /* Get executed before scoped_restore_current_thread above to apply
        still for the thread which has thrown the exception.  */
@@ -3752,7 +3940,7 @@  fetch_inferior_event (void *client_data)
 
     if (!ecs->wait_some_more)
       {
-	struct inferior *inf = find_inferior_ptid (ecs->ptid);
+	struct inferior *inf = find_inferior_ptid (ecs->target, ecs->ptid);
 	int should_stop = 1;
 	struct thread_info *thr = ecs->event_thread;
 
@@ -3854,23 +4042,28 @@  init_thread_stepping_state (struct thread_info *tss)
   tss->step_after_step_resume_breakpoint = 0;
 }
 
-/* Set the cached copy of the last ptid/waitstatus.  */
+/* Set the cached copy of the last target/ptid/waitstatus.  */
 
 void
-set_last_target_status (ptid_t ptid, struct target_waitstatus status)
+set_last_target_status (process_stratum_target *target, ptid_t ptid,
+			target_waitstatus status)
 {
+  target_last_proc_target = target;
   target_last_wait_ptid = ptid;
   target_last_waitstatus = status;
 }
 
-/* Return the cached copy of the last ptid/waitstatus returned
+/* Return the cached copy of the last target/ptid/waitstatus returned
    by target_wait()/deprecated_target_wait_hook().  The data is
    actually cached by handle_inferior_event(), which gets called
    immediately after target_wait()/deprecated_target_wait_hook().  */
 
 void
-get_last_target_status (ptid_t *ptid, struct target_waitstatus *status)
+get_last_target_status (process_stratum_target **target, ptid_t *ptid,
+			target_waitstatus *status)
 {
+  if (target != nullptr)
+    *target = target_last_proc_target;
   if (ptid != nullptr)
     *ptid = target_last_wait_ptid;
   if (status != nullptr)
@@ -3880,6 +4073,7 @@  get_last_target_status (ptid_t *ptid, struct target_waitstatus *status)
 void
 nullify_last_target_wait_ptid (void)
 {
+  target_last_proc_target = nullptr;
   target_last_wait_ptid = minus_one_ptid;
   target_last_waitstatus = {};
 }
@@ -3891,7 +4085,8 @@  context_switch (execution_control_state *ecs)
 {
   if (debug_infrun
       && ecs->ptid != inferior_ptid
-      && ecs->event_thread != inferior_thread ())
+      && (inferior_ptid == null_ptid
+	  || ecs->event_thread != inferior_thread ()))
     {
       fprintf_unfiltered (gdb_stdlog, "infrun: Switching context from %s ",
 			  target_pid_to_str (inferior_ptid).c_str ());
@@ -4177,20 +4372,19 @@  fill_in_stop_func (struct gdbarch *gdbarch,
 static enum stop_kind
 get_inferior_stop_soon (execution_control_state *ecs)
 {
-  struct inferior *inf = find_inferior_ptid (ecs->ptid);
+  struct inferior *inf = find_inferior_ptid (ecs->target, ecs->ptid);
 
   gdb_assert (inf != NULL);
   return inf->control.stop_soon;
 }
 
-/* Wait for one event.  Store the resulting waitstatus in WS, and
-   return the event ptid.  */
+/* Poll for one event out of the current target.  Store the resulting
+   waitstatus in WS, and return the event ptid.  Does not block.  */
 
 static ptid_t
-wait_one (struct target_waitstatus *ws)
+poll_one_curr_target (struct target_waitstatus *ws)
 {
   ptid_t event_ptid;
-  ptid_t wait_ptid = minus_one_ptid;
 
   overlay_cache_invalid = 1;
 
@@ -4201,16 +4395,101 @@  wait_one (struct target_waitstatus *ws)
   target_dcache_invalidate ();
 
   if (deprecated_target_wait_hook)
-    event_ptid = deprecated_target_wait_hook (wait_ptid, ws, 0);
+    event_ptid = deprecated_target_wait_hook (minus_one_ptid, ws, TARGET_WNOHANG);
   else
-    event_ptid = target_wait (wait_ptid, ws, 0);
+    event_ptid = target_wait (minus_one_ptid, ws, TARGET_WNOHANG);
 
   if (debug_infrun)
-    print_target_wait_results (wait_ptid, event_ptid, ws);
+    print_target_wait_results (minus_one_ptid, event_ptid, ws);
 
   return event_ptid;
 }
 
+/* An event reported by wait_one.  */
+
+struct wait_one_event
+{
+  /* The target the event came out of.  */
+  process_stratum_target *target;
+
+  /* The PTID the event was for.  */
+  ptid_t ptid;
+
+  /* The waitstatus.  */
+  target_waitstatus ws;
+};
+
+/* Wait for one event out of any target.  */
+
+static wait_one_event
+wait_one ()
+{
+  while (1)
+    {
+      for (inferior *inf : all_inferiors ())
+	{
+	  process_stratum_target *target = inf->process_target ();
+	  if (target == NULL
+	      || !target->is_async_p ()
+	      || !target->threads_executing)
+	    continue;
+
+	  switch_to_inferior_no_thread (inf);
+
+	  wait_one_event event;
+	  event.target = target;
+	  event.ptid = poll_one_curr_target (&event.ws);
+
+	  if (event.ws.kind == TARGET_WAITKIND_NO_RESUMED)
+	    {
+	      /* If nothing is resumed, remove the target from the
+		 event loop.  */
+	      target_async (0);
+	    }
+	  else if (event.ws.kind != TARGET_WAITKIND_IGNORE)
+	    return event;
+	}
+
+      /* Block waiting for some event.  */
+
+      fd_set readfds;
+      int nfds = 0;
+
+      FD_ZERO (&readfds);
+
+      for (inferior *inf : all_inferiors ())
+	{
+	  process_stratum_target *target = inf->process_target ();
+	  if (target == NULL
+	      || !target->is_async_p ()
+	      || !target->threads_executing)
+	    continue;
+
+	  int fd = target->async_wait_fd ();
+	  FD_SET (fd, &readfds);
+	  if (nfds <= fd)
+	    nfds = fd + 1;
+	}
+
+      if (nfds == 0)
+	{
+	  /* No waitable targets left.  All must be stopped.  */
+	  return {NULL, minus_one_ptid, {TARGET_WAITKIND_NO_RESUMED}};
+	}
+
+      QUIT;
+
+      int numfds = interruptible_select (nfds, &readfds, 0, NULL, 0);
+      if (numfds < 0)
+	{
+	  if (errno == EINTR)
+	    continue;
+	  else
+	    perror_with_name ("interruptible_select");
+	}
+    }
+}
+
 /* Generate a wrapper for target_stopped_by_REASON that works on PTID
    instead of the current thread.  */
 #define THREAD_STOPPED_BY(REASON)		\
@@ -4333,8 +4612,6 @@  stop_all_threads (void)
 			    "iterations=%d\n", pass, iterations);
       while (1)
 	{
-	  ptid_t event_ptid;
-	  struct target_waitstatus ws;
 	  int need_wait = 0;
 
 	  update_thread_list ();
@@ -4392,7 +4669,10 @@  stop_all_threads (void)
 	  if (pass > 0)
 	    pass = -1;
 
-	  event_ptid = wait_one (&ws);
+	  wait_one_event event = wait_one ();
+	  target_waitstatus &ws = event.ws;
+	  ptid_t &event_ptid = event.ptid;
+
 	  if (debug_infrun)
 	    {
 	      fprintf_unfiltered (gdb_stdlog,
@@ -4411,9 +4691,9 @@  stop_all_threads (void)
 	    }
 	  else
 	    {
-	      thread_info *t = find_thread_ptid (event_ptid);
+	      thread_info *t = find_thread_ptid (event.target, event_ptid);
 	      if (t == NULL)
-		t = add_thread (event_ptid);
+		t = add_thread (event.target, event_ptid);
 
 	      t->stop_requested = 0;
 	      t->executing = 0;
@@ -4422,7 +4702,7 @@  stop_all_threads (void)
 
 	      /* This may be the first time we see the inferior report
 		 a stop.  */
-	      inferior *inf = find_inferior_ptid (event_ptid);
+	      inferior *inf = find_inferior_ptid (event.target, event_ptid);
 	      if (inf->needs_setup)
 		{
 		  switch_to_thread_no_regs (t);
@@ -4572,7 +4852,7 @@  handle_no_resumed (struct execution_control_state *ecs)
      the synchronous command show "no unwaited-for " to the user.  */
   update_thread_list ();
 
-  for (thread_info *thread : all_non_exited_threads ())
+  for (thread_info *thread : all_non_exited_threads (ecs->target))
     {
       if (thread->executing
 	  || thread->suspend.waitstatus_pending_p)
@@ -4592,7 +4872,7 @@  handle_no_resumed (struct execution_control_state *ecs)
      process exited meanwhile (thus updating the thread list results
      in an empty thread list).  In this case we know we'll be getting
      a process exit event shortly.  */
-  for (inferior *inf : all_non_exited_inferiors ())
+  for (inferior *inf : all_non_exited_inferiors (ecs->target))
     {
       thread_info *thread = any_live_thread_of_inferior (inf);
       if (thread == NULL)
@@ -4662,8 +4942,8 @@  handle_inferior_event (struct execution_control_state *ecs)
       && handle_no_resumed (ecs))
     return;
 
-  /* Cache the last pid/waitstatus.  */
-  set_last_target_status (ecs->ptid, ecs->ws);
+  /* Cache the last target/ptid/waitstatus.  */
+  set_last_target_status (ecs->target, ecs->ptid, ecs->ws);
 
   /* Always clear state belonging to the previous time we stopped.  */
   stop_stack_dummy = STOP_NONE;
@@ -4680,10 +4960,10 @@  handle_inferior_event (struct execution_control_state *ecs)
   if (ecs->ws.kind != TARGET_WAITKIND_EXITED
       && ecs->ws.kind != TARGET_WAITKIND_SIGNALLED)
     {
-      ecs->event_thread = find_thread_ptid (ecs->ptid);
+      ecs->event_thread = find_thread_ptid (ecs->target, ecs->ptid);
       /* If it's a new thread, add it to the thread database.  */
       if (ecs->event_thread == NULL)
-	ecs->event_thread = add_thread (ecs->ptid);
+	ecs->event_thread = add_thread (ecs->target, ecs->ptid);
 
       /* Disable range stepping.  If the next step request could use a
 	 range, this will be end up re-enabled then.  */
@@ -4755,10 +5035,10 @@  handle_inferior_event (struct execution_control_state *ecs)
     else
       mark_ptid = ecs->ptid;
 
-    set_executing (mark_ptid, 0);
+    set_executing (ecs->target, mark_ptid, 0);
 
     /* Likewise the resumed flag.  */
-    set_resumed (mark_ptid, 0);
+    set_resumed (ecs->target, mark_ptid, 0);
   }
 
   switch (ecs->ws.kind)
@@ -4859,7 +5139,7 @@  handle_inferior_event (struct execution_control_state *ecs)
     case TARGET_WAITKIND_EXITED:
     case TARGET_WAITKIND_SIGNALLED:
       inferior_ptid = ecs->ptid;
-      set_current_inferior (find_inferior_ptid (ecs->ptid));
+      set_current_inferior (find_inferior_ptid (ecs->target, ecs->ptid));
       set_current_program_space (current_inferior ()->pspace);
       handle_vfork_child_exec_or_exit (0);
       target_terminal::ours ();	/* Must do this before mourn anyway.  */
@@ -4932,7 +5212,7 @@  Cannot fill $_exitsignal with the correct signal number.\n"));
 	if (displaced_step_in_progress_thread (ecs->event_thread))
 	  {
 	    struct inferior *parent_inf
-	      = find_inferior_ptid (ecs->ptid);
+	      = find_inferior_ptid (ecs->target, ecs->ptid);
 	    struct regcache *child_regcache;
 	    CORE_ADDR parent_pc;
 
@@ -4963,7 +5243,8 @@  Cannot fill $_exitsignal with the correct signal number.\n"));
 	       list yet at this point.  */
 
 	    child_regcache
-	      = get_thread_arch_aspace_regcache (ecs->ws.value.related_pid,
+	      = get_thread_arch_aspace_regcache (parent_inf->process_target (),
+						 ecs->ws.value.related_pid,
 						 gdbarch,
 						 parent_inf->aspace);
 	    /* Read PC value of parent process.  */
@@ -5031,10 +5312,16 @@  Cannot fill $_exitsignal with the correct signal number.\n"));
 
 	  ecs->event_thread->suspend.stop_signal = GDB_SIGNAL_0;
 
+	  process_stratum_target *targ
+	    = ecs->event_thread->inf->process_target ();
+
 	  should_resume = follow_fork ();
 
+	  /* Note that one of these may be an invalid pointer,
+	     depending on detach_fork.  */
 	  thread_info *parent = ecs->event_thread;
-	  thread_info *child = find_thread_ptid (ecs->ws.value.related_pid);
+	  thread_info *child
+	    = find_thread_ptid (targ, ecs->ws.value.related_pid);
 
 	  /* At this point, the parent is marked running, and the
 	     child is marked stopped.  */
@@ -5847,7 +6134,7 @@  handle_signal_stop (struct execution_control_state *ecs)
   if (random_signal)
     {
       /* Signal not for debugging purposes.  */
-      struct inferior *inf = find_inferior_ptid (ecs->ptid);
+      struct inferior *inf = find_inferior_ptid (ecs->target, ecs->ptid);
       enum gdb_signal stop_signal = ecs->event_thread->suspend.stop_signal;
 
       if (debug_infrun)
@@ -6896,7 +7183,8 @@  switch_back_to_stepped_thread (struct execution_control_state *ecs)
 	  /* Ignore threads of processes the caller is not
 	     resuming.  */
 	  if (!sched_multi
-	      && tp->ptid.pid () != ecs->ptid.pid ())
+	      && (tp->inf->process_target () != ecs->target
+		  || tp->inf->pid != ecs->ptid.pid ()))
 	    continue;
 
 	  /* When stepping over a breakpoint, we lock all threads
@@ -7842,7 +8130,7 @@  print_stop_event (struct ui_out *uiout, bool displays)
   struct target_waitstatus last;
   struct thread_info *tp;
 
-  get_last_target_status (nullptr, &last);
+  get_last_target_status (nullptr, nullptr, &last);
 
   {
     scoped_restore save_uiout = make_scoped_restore (&current_uiout, uiout);
@@ -7962,7 +8250,7 @@  normal_stop (void)
 {
   struct target_waitstatus last;
 
-  get_last_target_status (nullptr, &last);
+  get_last_target_status (nullptr, nullptr, &last);
 
   new_stop_id ();
 
@@ -7971,10 +8259,10 @@  normal_stop (void)
      frontend/user running state.  A QUIT is an easy exception to see
      here, so do this before any filtered output.  */
 
-  gdb::optional<scoped_finish_thread_state> maybe_finish_thread_state;
+  ptid_t finish_ptid = null_ptid;
 
   if (!non_stop)
-    maybe_finish_thread_state.emplace (minus_one_ptid);
+    finish_ptid = minus_one_ptid;
   else if (last.kind == TARGET_WAITKIND_SIGNALLED
 	   || last.kind == TARGET_WAITKIND_EXITED)
     {
@@ -7984,10 +8272,17 @@  normal_stop (void)
 	 linux-fork.c automatically switches to another fork from
 	 within target_mourn_inferior.  */
       if (inferior_ptid != null_ptid)
-	maybe_finish_thread_state.emplace (ptid_t (inferior_ptid.pid ()));
+	finish_ptid = ptid_t (inferior_ptid.pid ());
     }
   else if (last.kind != TARGET_WAITKIND_NO_RESUMED)
-    maybe_finish_thread_state.emplace (inferior_ptid);
+    finish_ptid = inferior_ptid;
+
+  gdb::optional<scoped_finish_thread_state> maybe_finish_thread_state;
+  if (finish_ptid != null_ptid)
+    {
+      maybe_finish_thread_state.emplace
+	(user_visible_resume_target (finish_ptid), finish_ptid);
+    }
 
   /* As we're presenting a stop, and potentially removing breakpoints,
      update the thread list so we can tell whether there are threads
diff --git a/gdb/infrun.h b/gdb/infrun.h
index 042edbbb66..2412d26123 100644
--- a/gdb/infrun.h
+++ b/gdb/infrun.h
@@ -25,6 +25,7 @@  struct target_waitstatus;
 struct frame_info;
 struct address_space;
 struct return_value_info;
+struct process_stratum_target;
 
 /* True if we are debugging run control.  */
 extern unsigned int debug_infrun;
@@ -93,7 +94,13 @@  extern void proceed (CORE_ADDR, enum gdb_signal);
    resumed.  */
 extern ptid_t user_visible_resume_ptid (int step);
 
-extern void wait_for_inferior (void);
+/* Return the process_stratum target that we will proceed, in the
+   perspective of the user/frontend.  If RESUME_PTID is
+   MINUS_ONE_PTID, then we'll resume all threads of all targets, so
+   the function returns NULL.  Otherwise, we'll be resuming a process
+   or thread of the current process, so we return the current
+   inferior's process stratum target.  */
+extern process_stratum_target *user_visible_resume_target (ptid_t resume_ptid);
 
 /* Return control to GDB when the inferior stops for real.  Print
    appropriate messages, remove breakpoints, give terminal our modes,
@@ -101,10 +108,11 @@  extern void wait_for_inferior (void);
    target, false otherwise.  */
 extern int normal_stop (void);
 
-extern void get_last_target_status (ptid_t *ptid,
+extern void get_last_target_status (process_stratum_target **target,
+				    ptid_t *ptid,
 				    struct target_waitstatus *status);
 
-extern void set_last_target_status (ptid_t ptid,
+extern void set_last_target_status (process_stratum_target *target, ptid_t ptid,
 				    struct target_waitstatus status);
 
 extern void nullify_last_target_wait_ptid ();
diff --git a/gdb/inline-frame.c b/gdb/inline-frame.c
index b039ab8fa2..c4958518e5 100644
--- a/gdb/inline-frame.c
+++ b/gdb/inline-frame.c
@@ -96,37 +96,54 @@  find_inline_frame_state (thread_info *thread)
   return &state;
 }
 
-/* Forget about any hidden inlined functions in PTID, which is new or
-   about to be resumed.  PTID may be minus_one_ptid (all processes)
-   or a PID (all threads in this process).  */
+/* See inline-frame.h.  */
 
 void
-clear_inline_frame_state (ptid_t ptid)
+clear_inline_frame_state (process_stratum_target *target, ptid_t filter_ptid)
 {
-  if (ptid == minus_one_ptid)
-    {
-      inline_states.clear ();
-      return;
-    }
+  gdb_assert (target != NULL);
 
-  if (ptid.is_pid ())
+  if (filter_ptid == minus_one_ptid || filter_ptid.is_pid ())
     {
-      int pid = ptid.pid ();
+      auto matcher = [target, &filter_ptid] (const inline_state &state)
+	{
+	  thread_info *t = state.thread;
+	  return (t->inf->process_target () == target
+		  && t->ptid.matches (filter_ptid));
+	};
+
       auto it = std::remove_if (inline_states.begin (), inline_states.end (),
-				[pid] (const inline_state &state)
-				  {
-				    return pid == state.thread->inf->pid;
-				  });
+				matcher);
 
       inline_states.erase (it, inline_states.end ());
 
       return;
     }
 
+
+  auto matcher = [target, &filter_ptid] (const inline_state &state)
+    {
+      thread_info *t = state.thread;
+      return (t->inf->process_target () == target
+	      && filter_ptid == t->ptid);
+    };
+
+  auto it = std::find_if (inline_states.begin (), inline_states.end (),
+			  matcher);
+
+  if (it != inline_states.end ())
+    unordered_remove (inline_states, it);
+}
+
+/* See inline-frame.h.  */
+
+void
+clear_inline_frame_state (thread_info *thread)
+{
   auto it = std::find_if (inline_states.begin (), inline_states.end (),
-			  [&ptid] (const inline_state &state)
+			  [thread] (const inline_state &state)
 			    {
-			      return ptid == state.thread->ptid;
+			      return thread == state.thread;
 			    });
 
   if (it != inline_states.end ())
diff --git a/gdb/inline-frame.h b/gdb/inline-frame.h
index e1d98f5ab5..6ba748ee22 100644
--- a/gdb/inline-frame.h
+++ b/gdb/inline-frame.h
@@ -23,6 +23,7 @@ 
 struct frame_info;
 struct frame_unwind;
 struct bpstats;
+struct process_stratum_target;
 
 /* The inline frame unwinder.  */
 
@@ -39,10 +40,15 @@  extern const struct frame_unwind inline_frame_unwind;
 void skip_inline_frames (thread_info *thread, struct bpstats *stop_chain);
 
 /* Forget about any hidden inlined functions in PTID, which is new or
-   about to be resumed.  If PTID is minus_one_ptid, forget about all
-   hidden inlined functions.  */
+   about to be resumed.  PTID may be minus_one_ptid (all processes of
+   TARGET) or a PID (all threads in this process of TARGET).  */
 
-void clear_inline_frame_state (ptid_t ptid);
+void clear_inline_frame_state (process_stratum_target *target, ptid_t ptid);
+
+/* Forget about any hidden inlined functions in THREAD, which is new
+   or about to be resumed.  */
+
+void clear_inline_frame_state (thread_info *thread);
 
 /* Step into an inlined function by unhiding it.  */
 
diff --git a/gdb/linux-fork.c b/gdb/linux-fork.c
index ab96be2f38..d3e094f530 100644
--- a/gdb/linux-fork.c
+++ b/gdb/linux-fork.c
@@ -519,7 +519,7 @@  Please switch to another checkpoint before deleting the current one"));
      list, waitpid the ptid.
      If fi->parent_ptid is a part of lwp and it is stopped, waitpid the
      ptid.  */
-  thread_info *parent = find_thread_ptid (pptid);
+  thread_info *parent = find_thread_ptid (linux_target, pptid);
   if ((parent == NULL && find_fork_ptid (pptid))
       || (parent != NULL && parent->state == THREAD_STOPPED))
     {
@@ -679,7 +679,7 @@  checkpoint_command (const char *args, int from_tty)
     error (_("checkpoint: call_function_by_hand returned null."));
 
   retpid = value_as_long (ret);
-  get_last_target_status (&last_target_ptid, &last_target_waitstatus);
+  get_last_target_status (nullptr, &last_target_ptid, &last_target_waitstatus);
 
   fp = find_fork_pid (retpid);
 
diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index 8f7d4b6eba..89a0b9378f 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -518,9 +518,12 @@  linux_nat_target::follow_fork (int follow_child, int detach_fork)
 	}
       else
 	{
-	  scoped_restore save_inferior_ptid
-	    = make_scoped_restore (&inferior_ptid);
-	  inferior_ptid = child_ptid;
+	  /* Switching inferior_ptid is not enough, because then
+	     inferior_thread () would crash by not finding the thread
+	     in the current inferior.  */
+	  scoped_restore_current_thread restore_current_thread;
+	  thread_info *child = find_thread_ptid (this, child_ptid);
+	  switch_to_thread (child);
 
 	  /* Let the thread_db layer learn about this new process.  */
 	  check_for_thread_db ();
@@ -992,7 +995,7 @@  linux_nat_switch_fork (ptid_t new_ptid)
   /* This changes the thread's ptid while preserving the gdb thread
      num.  Also changes the inferior pid, while preserving the
      inferior num.  */
-  thread_change_ptid (inferior_ptid, new_ptid);
+  thread_change_ptid (linux_target, inferior_ptid, new_ptid);
 
   /* We've just told GDB core that the thread changed target id, but,
      in fact, it really is a different thread, with different register
@@ -1005,7 +1008,7 @@  linux_nat_switch_fork (ptid_t new_ptid)
 static void
 exit_lwp (struct lwp_info *lp)
 {
-  struct thread_info *th = find_thread_ptid (lp->ptid);
+  struct thread_info *th = find_thread_ptid (linux_target, lp->ptid);
 
   if (th)
     {
@@ -1165,9 +1168,9 @@  attach_proc_task_lwp_callback (ptid_t ptid)
 	  /* Also add the LWP to gdb's thread list, in case a
 	     matching libthread_db is not found (or the process uses
 	     raw clone).  */
-	  add_thread (lp->ptid);
-	  set_running (lp->ptid, 1);
-	  set_executing (lp->ptid, 1);
+	  add_thread (linux_target, lp->ptid);
+	  set_running (linux_target, lp->ptid, 1);
+	  set_executing (linux_target, lp->ptid, 1);
 	}
 
       return 1;
@@ -1206,7 +1209,7 @@  linux_nat_target::attach (const char *args, int from_tty)
   ptid = ptid_t (inferior_ptid.pid (),
 		 inferior_ptid.pid (),
 		 0);
-  thread_change_ptid (inferior_ptid, ptid);
+  thread_change_ptid (linux_target, inferior_ptid, ptid);
 
   /* Add the initial process as the first LWP to the list.  */
   lp = add_initial_lwp (ptid);
@@ -1307,7 +1310,7 @@  get_detach_signal (struct lwp_info *lp)
     signo = gdb_signal_from_host (WSTOPSIG (lp->status));
   else
     {
-      struct thread_info *tp = find_thread_ptid (lp->ptid);
+      struct thread_info *tp = find_thread_ptid (linux_target, lp->ptid);
 
       if (target_is_non_stop_p () && !tp->executing)
 	{
@@ -1319,10 +1322,12 @@  get_detach_signal (struct lwp_info *lp)
       else if (!target_is_non_stop_p ())
 	{
 	  ptid_t last_ptid;
+	  process_stratum_target *last_target;
 
-	  get_last_target_status (&last_ptid, nullptr);
+	  get_last_target_status (&last_target, &last_ptid, nullptr);
 
-	  if (lp->ptid.lwp () == last_ptid.lwp ())
+	  if (last_target == linux_target
+	      && lp->ptid.lwp () == last_ptid.lwp ())
 	    signo = tp->suspend.stop_signal;
 	}
     }
@@ -1519,7 +1524,7 @@  linux_resume_one_lwp_throw (struct lwp_info *lp, int step,
      handle the case of stepping a breakpoint instruction).  */
   if (step)
     {
-      struct regcache *regcache = get_thread_regcache (lp->ptid);
+      struct regcache *regcache = get_thread_regcache (linux_target, lp->ptid);
 
       lp->stop_pc = regcache_read_pc (regcache);
     }
@@ -1538,7 +1543,7 @@  linux_resume_one_lwp_throw (struct lwp_info *lp, int step,
   lp->stopped = 0;
   lp->core = -1;
   lp->stop_reason = TARGET_STOPPED_BY_NO_REASON;
-  registers_changed_ptid (lp->ptid);
+  registers_changed_ptid (linux_target, lp->ptid);
 }
 
 /* Called when we try to resume a stopped LWP and that errors out.  If
@@ -1597,7 +1602,7 @@  resume_lwp (struct lwp_info *lp, int step, enum gdb_signal signo)
 {
   if (lp->stopped)
     {
-      struct inferior *inf = find_inferior_ptid (lp->ptid);
+      struct inferior *inf = find_inferior_ptid (linux_target, lp->ptid);
 
       if (inf->vfork_child != NULL)
 	{
@@ -1651,7 +1656,7 @@  linux_nat_resume_callback (struct lwp_info *lp, struct lwp_info *except)
     {
       struct thread_info *thread;
 
-      thread = find_thread_ptid (lp->ptid);
+      thread = find_thread_ptid (linux_target, lp->ptid);
       if (thread != NULL)
 	{
 	  signo = thread->suspend.stop_signal;
@@ -1808,7 +1813,7 @@  linux_handle_syscall_trap (struct lwp_info *lp, int stopping)
 {
   struct target_waitstatus *ourstatus = &lp->waitstatus;
   struct gdbarch *gdbarch = target_thread_architecture (lp->ptid);
-  thread_info *thread = find_thread_ptid (lp->ptid);
+  thread_info *thread = find_thread_ptid (linux_target, lp->ptid);
   int syscall_number = (int) gdbarch_get_syscall_number (gdbarch, thread);
 
   if (stopping)
@@ -2028,15 +2033,15 @@  linux_handle_extended_wait (struct lwp_info *lp, int status)
 	      /* The process is not using thread_db.  Add the LWP to
 		 GDB's list.  */
 	      target_post_attach (new_lp->ptid.lwp ());
-	      add_thread (new_lp->ptid);
+	      add_thread (linux_target, new_lp->ptid);
 	    }
 
 	  /* Even if we're stopping the thread for some reason
 	     internal to this module, from the perspective of infrun
 	     and the user/frontend, this new thread is running until
 	     it next reports a stop.  */
-	  set_running (new_lp->ptid, 1);
-	  set_executing (new_lp->ptid, 1);
+	  set_running (linux_target, new_lp->ptid, 1);
+	  set_executing (linux_target, new_lp->ptid, 1);
 
 	  if (WSTOPSIG (status) != SIGSTOP)
 	    {
@@ -2259,7 +2264,7 @@  wait_lwp (struct lwp_info *lp)
 
   if (lp->must_set_ptrace_flags)
     {
-      struct inferior *inf = find_inferior_pid (lp->ptid.pid ());
+      inferior *inf = find_inferior_pid (linux_target, lp->ptid.pid ());
       int options = linux_nat_ptrace_options (inf->attach_flag);
 
       linux_enable_event_reporting (lp->ptid.lwp (), options);
@@ -2486,7 +2491,7 @@  linux_nat_target::low_status_is_event (int status)
 static int
 stop_wait_callback (struct lwp_info *lp)
 {
-  struct inferior *inf = find_inferior_ptid (lp->ptid);
+  inferior *inf = find_inferior_ptid (linux_target, lp->ptid);
 
   /* If this is a vfork parent, bail out, it is not going to report
      any SIGSTOP until the vfork is done with.  */
@@ -2579,7 +2584,7 @@  status_callback (struct lwp_info *lp)
   if (lp->stop_reason == TARGET_STOPPED_BY_SW_BREAKPOINT
       || lp->stop_reason == TARGET_STOPPED_BY_HW_BREAKPOINT)
     {
-      struct regcache *regcache = get_thread_regcache (lp->ptid);
+      struct regcache *regcache = get_thread_regcache (linux_target, lp->ptid);
       CORE_ADDR pc;
       int discard = 0;
 
@@ -2700,7 +2705,7 @@  save_stop_reason (struct lwp_info *lp)
   if (!linux_target->low_status_is_event (lp->status))
     return;
 
-  regcache = get_thread_regcache (lp->ptid);
+  regcache = get_thread_regcache (linux_target, lp->ptid);
   gdbarch = regcache->arch ();
 
   pc = regcache_read_pc (regcache);
@@ -2962,7 +2967,7 @@  linux_nat_filter_event (int lwpid, int status)
       lp = add_lwp (ptid_t (lwpid, lwpid, 0));
       lp->stopped = 1;
       lp->resumed = 1;
-      add_thread (lp->ptid);
+      add_thread (linux_target, lp->ptid);
     }
 
   if (WIFSTOPPED (status) && !lp)
@@ -2988,7 +2993,7 @@  linux_nat_filter_event (int lwpid, int status)
 
   if (WIFSTOPPED (status) && lp->must_set_ptrace_flags)
     {
-      struct inferior *inf = find_inferior_pid (lp->ptid.pid ());
+      inferior *inf = find_inferior_pid (linux_target, lp->ptid.pid ());
       int options = linux_nat_ptrace_options (inf->attach_flag);
 
       linux_enable_event_reporting (lp->ptid.lwp (), options);
@@ -3270,7 +3275,7 @@  linux_nat_wait_1 (ptid_t ptid, struct target_waitstatus *ourstatus,
   if (inferior_ptid.is_pid ())
     {
       /* Upgrade the main thread's ptid.  */
-      thread_change_ptid (inferior_ptid,
+      thread_change_ptid (linux_target, inferior_ptid,
 			  ptid_t (inferior_ptid.pid (),
 				  inferior_ptid.pid (), 0));
 
@@ -3415,7 +3420,7 @@  linux_nat_wait_1 (ptid_t ptid, struct target_waitstatus *ourstatus,
   if (lp->stop_reason == TARGET_STOPPED_BY_SW_BREAKPOINT
       && !USE_SIGTRAP_SIGINFO)
     {
-      struct regcache *regcache = get_thread_regcache (lp->ptid);
+      struct regcache *regcache = get_thread_regcache (linux_target, lp->ptid);
       struct gdbarch *gdbarch = regcache->arch ();
       int decr_pc = gdbarch_decr_pc_after_break (gdbarch);
 
@@ -3516,7 +3521,7 @@  resume_stopped_resumed_lwps (struct lwp_info *lp, const ptid_t wait_ptid)
     }
   else
     {
-      struct regcache *regcache = get_thread_regcache (lp->ptid);
+      struct regcache *regcache = get_thread_regcache (linux_target, lp->ptid);
       struct gdbarch *gdbarch = regcache->arch ();
 
       try
@@ -4389,6 +4394,12 @@  linux_async_pipe (int enable)
   return previous;
 }
 
+int
+linux_nat_target::async_wait_fd ()
+{
+  return linux_nat_event_pipe[0];
+}
+
 /* target_async implementation.  */
 
 void
@@ -4446,7 +4457,7 @@  linux_nat_stop_lwp (struct lwp_info *lwp)
 
       if (debug_linux_nat)
 	{
-	  if (find_thread_ptid (lwp->ptid)->stop_requested)
+	  if (find_thread_ptid (linux_target, lwp->ptid)->stop_requested)
 	    fprintf_unfiltered (gdb_stdlog,
 				"LNSL: already stopped/stop_requested %s\n",
 				target_pid_to_str (lwp->ptid).c_str ());
@@ -4503,7 +4514,7 @@  linux_nat_target::thread_address_space (ptid_t ptid)
       pid = ptid.pid ();
     }
 
-  inf = find_inferior_pid (pid);
+  inf = find_inferior_pid (this, pid);
   gdb_assert (inf != NULL);
   return inf->aspace;
 }
diff --git a/gdb/linux-nat.h b/gdb/linux-nat.h
index 0c1695ad10..3c1045f3a4 100644
--- a/gdb/linux-nat.h
+++ b/gdb/linux-nat.h
@@ -88,6 +88,7 @@  public:
   bool supports_non_stop () override;
   bool always_non_stop_p () override;
 
+  int async_wait_fd () override;
   void async (int) override;
 
   void close () override;
diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index 7d0c3e9fc9..025cdb470a 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -1717,7 +1717,8 @@  linux_corefile_thread (struct thread_info *info,
 {
   struct regcache *regcache;
 
-  regcache = get_thread_arch_regcache (info->ptid, args->gdbarch);
+  regcache = get_thread_arch_regcache (info->inf->process_target (),
+				       info->ptid, args->gdbarch);
 
   target_fetch_registers (regcache, -1);
   gdb::byte_vector siginfo_data = linux_get_siginfo_data (info, args->gdbarch);
diff --git a/gdb/linux-thread-db.c b/gdb/linux-thread-db.c
index b2c9d6d77c..a3a71c742b 100644
--- a/gdb/linux-thread-db.c
+++ b/gdb/linux-thread-db.c
@@ -167,6 +167,9 @@  struct thread_db_info
 {
   struct thread_db_info *next;
 
+  /* The target this thread_db_info is bound to.  */
+  process_stratum_target *process_target;
+
   /* Process id this object refers to.  */
   int pid;
 
@@ -227,6 +230,7 @@  add_thread_db_info (void *handle)
 {
   struct thread_db_info *info = XCNEW (struct thread_db_info);
 
+  info->process_target = current_inferior ()->process_target ();
   info->pid = inferior_ptid.pid ();
   info->handle = handle;
 
@@ -245,12 +249,12 @@  add_thread_db_info (void *handle)
    related to process PID, if any; NULL otherwise.  */
 
 static struct thread_db_info *
-get_thread_db_info (int pid)
+get_thread_db_info (process_stratum_target *targ, int pid)
 {
   struct thread_db_info *info;
 
   for (info = thread_db_list; info; info = info->next)
-    if (pid == info->pid)
+    if (targ == info->process_target && pid == info->pid)
       return info;
 
   return NULL;
@@ -264,14 +268,14 @@  static const char *thread_db_err_str (td_err_e err);
    LIBTHREAD_DB_SO's dlopen'ed handle.  */
 
 static void
-delete_thread_db_info (int pid)
+delete_thread_db_info (process_stratum_target *targ, int pid)
 {
   struct thread_db_info *info, *info_prev;
 
   info_prev = NULL;
 
   for (info = thread_db_list; info; info_prev = info, info = info->next)
-    if (pid == info->pid)
+    if (targ == info->process_target && pid == info->pid)
       break;
 
   if (info == NULL)
@@ -405,7 +409,7 @@  thread_from_lwp (thread_info *stopped, ptid_t ptid)
      LWP.  */
   gdb_assert (ptid.lwp () != 0);
 
-  info = get_thread_db_info (ptid.pid ());
+  info = get_thread_db_info (stopped->inf->process_target (), ptid.pid ());
 
   /* Access an lwp we know is stopped.  */
   info->proc_handle.thread = stopped;
@@ -421,7 +425,7 @@  thread_from_lwp (thread_info *stopped, ptid_t ptid)
 	   thread_db_err_str (err));
 
   /* Fill the cache.  */
-  tp = find_thread_ptid (ptid);
+  tp = find_thread_ptid (stopped->inf->process_target (), ptid);
   return record_thread (info, tp, ptid, &th, &ti);
 }
 
@@ -433,12 +437,12 @@  thread_db_notice_clone (ptid_t parent, ptid_t child)
 {
   struct thread_db_info *info;
 
-  info = get_thread_db_info (child.pid ());
+  info = get_thread_db_info (linux_target, child.pid ());
 
   if (info == NULL)
     return 0;
 
-  thread_info *stopped = find_thread_ptid (parent);
+  thread_info *stopped = find_thread_ptid (linux_target, parent);
 
   thread_from_lwp (stopped, child);
 
@@ -683,13 +687,13 @@  check_thread_db_callback (const td_thrhandle_t *th, void *arg)
      to how GDB accesses TLS could result in this passing
      without exercising the calls it's supposed to.  */
   ptid_t ptid = ptid_t (tdb_testinfo->info->pid, ti.ti_lid, 0);
-  struct thread_info *thread_info = find_thread_ptid (ptid);
+  thread_info *thread_info = find_thread_ptid (linux_target, ptid);
   if (thread_info != NULL && thread_info->priv != NULL)
     {
       LOG ("; errno");
 
       scoped_restore_current_thread restore_current_thread;
-      switch_to_thread (ptid);
+      switch_to_thread (thread_info);
 
       expression_up expr = parse_expression ("(int) errno");
       struct value *val = evaluate_expression (expr.get ());
@@ -938,10 +942,8 @@  try_thread_db_load_1 (struct thread_db_info *info)
     }
 
   /* The thread library was detected.  Activate the thread_db target
-     if this is the first process using it.  */
-  if (thread_db_list->next == NULL)
-    push_target (&the_thread_db_target);
-
+     for this process.  */
+  push_target (&the_thread_db_target);
   return true;
 }
 
@@ -1011,7 +1013,8 @@  try_thread_db_load (const char *library, bool check_auto_load_safe)
     return true;
 
   /* This library "refused" to work on current inferior.  */
-  delete_thread_db_info (inferior_ptid.pid ());
+  delete_thread_db_info (current_inferior ()->process_target (),
+			 inferior_ptid.pid ());
   return false;
 }
 
@@ -1179,7 +1182,8 @@  thread_db_load (void)
 {
   struct thread_db_info *info;
 
-  info = get_thread_db_info (inferior_ptid.pid ());
+  info = get_thread_db_info (current_inferior ()->process_target (),
+			     inferior_ptid.pid ());
 
   if (info != NULL)
     return true;
@@ -1346,7 +1350,7 @@  record_thread (struct thread_db_info *info,
      thread with this PTID, but it's marked exited, then the kernel
      reused the tid of an old thread.  */
   if (tp == NULL || tp->state == THREAD_EXITED)
-    tp = add_thread_with_info (ptid, priv);
+    tp = add_thread_with_info (info->process_target, ptid, priv);
   else
     tp->priv.reset (priv);
 
@@ -1359,16 +1363,14 @@  record_thread (struct thread_db_info *info,
 void
 thread_db_target::detach (inferior *inf, int from_tty)
 {
-  delete_thread_db_info (inf->pid);
+  delete_thread_db_info (inf->process_target (), inf->pid);
 
   beneath ()->detach (inf, from_tty);
 
   /* NOTE: From this point on, inferior_ptid is null_ptid.  */
 
-  /* If there are no more processes using libpthread, detach the
-     thread_db target ops.  */
-  if (!thread_db_list)
-    unpush_target (this);
+  /* Detach the thread_db target from this inferior.  */
+  unpush_target (this);
 }
 
 ptid_t
@@ -1377,7 +1379,10 @@  thread_db_target::wait (ptid_t ptid, struct target_waitstatus *ourstatus,
 {
   struct thread_db_info *info;
 
-  ptid = beneath ()->wait (ptid, ourstatus, options);
+  process_stratum_target *beneath
+    = as_process_stratum_target (this->beneath ());
+
+  ptid = beneath->wait (ptid, ourstatus, options);
 
   switch (ourstatus->kind)
     {
@@ -1388,7 +1393,7 @@  thread_db_target::wait (ptid_t ptid, struct target_waitstatus *ourstatus,
       return ptid;
     }
 
-  info = get_thread_db_info (ptid.pid ());
+  info = get_thread_db_info (beneath, ptid.pid ());
 
   /* If this process isn't using thread_db, we're done.  */
   if (info == NULL)
@@ -1398,15 +1403,14 @@  thread_db_target::wait (ptid_t ptid, struct target_waitstatus *ourstatus,
     {
       /* New image, it may or may not end up using thread_db.  Assume
 	 not unless we find otherwise.  */
-      delete_thread_db_info (ptid.pid ());
-      if (!thread_db_list)
-	unpush_target (&the_thread_db_target);
+      delete_thread_db_info (beneath, ptid.pid ());
+      unpush_target (this);
 
       return ptid;
     }
 
   /* Fill in the thread's user-level thread id and status.  */
-  thread_from_lwp (find_thread_ptid (ptid), ptid);
+  thread_from_lwp (find_thread_ptid (beneath, ptid), ptid);
 
   return ptid;
 }
@@ -1414,13 +1418,15 @@  thread_db_target::wait (ptid_t ptid, struct target_waitstatus *ourstatus,
 void
 thread_db_target::mourn_inferior ()
 {
-  delete_thread_db_info (inferior_ptid.pid ());
+  process_stratum_target *target_beneath
+    = as_process_stratum_target (this->beneath ());
+
+  delete_thread_db_info (target_beneath, inferior_ptid.pid ());
 
-  beneath ()->mourn_inferior ();
+  target_beneath->mourn_inferior ();
 
-  /* Detach thread_db target ops.  */
-  if (!thread_db_list)
-    unpush_target (&the_thread_db_target);
+  /* Detach the thread_db target from this inferior.  */
+  unpush_target (this);
 }
 
 struct callback_data
@@ -1485,7 +1491,7 @@  find_new_threads_callback (const td_thrhandle_t *th_p, void *data)
     }
 
   ptid_t ptid (info->pid, ti.ti_lid);
-  tp = find_thread_ptid (ptid);
+  tp = find_thread_ptid (info->process_target, ptid);
   if (tp == NULL || tp->priv == NULL)
     record_thread (info, tp, ptid, th_p, &ti);
 
@@ -1552,7 +1558,8 @@  thread_db_find_new_threads_2 (thread_info *stopped, bool until_no_new)
   struct thread_db_info *info;
   int i, loop;
 
-  info = get_thread_db_info (stopped->ptid.pid ());
+  info = get_thread_db_info (stopped->inf->process_target (),
+			     stopped->ptid.pid ());
 
   /* Access an lwp we know is stopped.  */
   info->proc_handle.thread = stopped;
@@ -1595,16 +1602,14 @@  thread_db_target::update_thread_list ()
 
   for (inferior *inf : all_inferiors ())
     {
-      struct thread_info *thread;
-
       if (inf->pid == 0)
 	continue;
 
-      info = get_thread_db_info (inf->pid);
+      info = get_thread_db_info (inf->process_target (), inf->pid);
       if (info == NULL)
 	continue;
 
-      thread = any_live_thread_of_inferior (inf);
+      thread_info *thread = any_live_thread_of_inferior (inf);
       if (thread == NULL || thread->executing)
 	continue;
 
@@ -1632,7 +1637,7 @@  thread_db_target::update_thread_list ()
 std::string
 thread_db_target::pid_to_str (ptid_t ptid)
 {
-  struct thread_info *thread_info = find_thread_ptid (ptid);
+  thread_info *thread_info = find_thread_ptid (current_inferior (), ptid);
 
   if (thread_info != NULL && thread_info->priv != NULL)
     {
@@ -1725,9 +1730,10 @@  thread_db_target::get_thread_local_address (ptid_t ptid,
 					    CORE_ADDR offset)
 {
   struct thread_info *thread_info;
-
+  process_stratum_target *beneath
+    = as_process_stratum_target (this->beneath ());
   /* Find the matching thread.  */
-  thread_info = find_thread_ptid (ptid);
+  thread_info = find_thread_ptid (beneath, ptid);
 
   /* We may not have discovered the thread yet.  */
   if (thread_info != NULL && thread_info->priv == NULL)
@@ -1737,7 +1743,7 @@  thread_db_target::get_thread_local_address (ptid_t ptid,
     {
       td_err_e err;
       psaddr_t address;
-      thread_db_info *info = get_thread_db_info (ptid.pid ());
+      thread_db_info *info = get_thread_db_info (beneath, ptid.pid ());
       thread_db_thread_info *priv = get_thread_db_thread_info (thread_info);
 
       /* Finally, get the address of the variable.  */
@@ -1796,7 +1802,7 @@  thread_db_target::get_thread_local_address (ptid_t ptid,
 	      : (CORE_ADDR) (uintptr_t) address);
     }
 
-  return beneath ()->get_thread_local_address (ptid, lm, offset);
+  return beneath->get_thread_local_address (ptid, lm, offset);
 }
 
 /* Implement the to_get_ada_task_ptid target method for this target.  */
@@ -1811,12 +1817,13 @@  thread_db_target::get_ada_task_ptid (long lwp, long thread)
 void
 thread_db_target::resume (ptid_t ptid, int step, enum gdb_signal signo)
 {
-  struct thread_db_info *info;
+  process_stratum_target *beneath
+    = as_process_stratum_target (this->beneath ());
 
-  if (ptid == minus_one_ptid)
-    info = get_thread_db_info (inferior_ptid.pid ());
-  else
-    info = get_thread_db_info (ptid.pid ());
+  thread_db_info *info
+    = get_thread_db_info (beneath, (ptid == minus_one_ptid
+				    ? inferior_ptid.pid ()
+				    : ptid.pid ()));
 
   /* This workaround is only needed for child fork lwps stopped in a
      PTRACE_O_TRACEFORK event.  When the inferior is resumed, the
@@ -1824,7 +1831,7 @@  thread_db_target::resume (ptid_t ptid, int step, enum gdb_signal signo)
   if (info)
     info->need_stale_parent_threads_check = 0;
 
-  beneath ()->resume (ptid, step, signo);
+  beneath->resume (ptid, step, signo);
 }
 
 /* std::sort helper function for info_auto_load_libthread_db, sort the
@@ -1950,7 +1957,8 @@  maintenance_check_libthread_db (const char *args, int from_tty)
   if (inferior_pid == 0)
     error (_("No inferior running"));
 
-  info = get_thread_db_info (inferior_pid);
+  info = get_thread_db_info (current_inferior ()->process_target (),
+			     inferior_pid);
   if (info == NULL)
     error (_("No libthread_db loaded"));
 
diff --git a/gdb/mi/mi-interp.c b/gdb/mi/mi-interp.c
index 9033a61950..1a9c13c0dc 100644
--- a/gdb/mi/mi-interp.c
+++ b/gdb/mi/mi-interp.c
@@ -958,7 +958,8 @@  multiple_inferiors_p ()
 }
 
 static void
-mi_on_resume_1 (struct mi_interp *mi, ptid_t ptid)
+mi_on_resume_1 (struct mi_interp *mi,
+		process_stratum_target *targ, ptid_t ptid)
 {
   /* To cater for older frontends, emit ^running, but do it only once
      per each command.  We do it here, since at this point we know
@@ -981,7 +982,7 @@  mi_on_resume_1 (struct mi_interp *mi, ptid_t ptid)
       && !multiple_inferiors_p ())
     fprintf_unfiltered (mi->raw_stdout, "*running,thread-id=\"all\"\n");
   else
-    for (thread_info *tp : all_non_exited_threads (ptid))
+    for (thread_info *tp : all_non_exited_threads (targ, ptid))
       mi_output_running (tp);
 
   if (!running_result_record_printed && mi_proceeded)
@@ -1001,10 +1002,11 @@  mi_on_resume (ptid_t ptid)
 {
   struct thread_info *tp = NULL;
 
+  process_stratum_target *target = current_inferior ()->process_target ();
   if (ptid == minus_one_ptid || ptid.is_pid ())
     tp = inferior_thread ();
   else
-    tp = find_thread_ptid (ptid);
+    tp = find_thread_ptid (target, ptid);
 
   /* Suppress output while calling an inferior function.  */
   if (tp->control.in_infcall)
@@ -1020,7 +1022,7 @@  mi_on_resume (ptid_t ptid)
       target_terminal::scoped_restore_terminal_state term_state;
       target_terminal::ours_for_output ();
 
-      mi_on_resume_1 (mi, ptid);
+      mi_on_resume_1 (mi, target, ptid);
     }
 }
 
diff --git a/gdb/nat/fork-inferior.c b/gdb/nat/fork-inferior.c
index 355e9bef43..3d9f0881c0 100644
--- a/gdb/nat/fork-inferior.c
+++ b/gdb/nat/fork-inferior.c
@@ -450,7 +450,7 @@  fork_inferior (const char *exec_file_arg, const std::string &allargs,
 /* See nat/fork-inferior.h.  */
 
 ptid_t
-startup_inferior (pid_t pid, int ntraps,
+startup_inferior (process_stratum_target *proc_target, pid_t pid, int ntraps,
 		  struct target_waitstatus *last_waitstatus,
 		  ptid_t *last_ptid)
 {
@@ -502,7 +502,7 @@  startup_inferior (pid_t pid, int ntraps,
 	  case TARGET_WAITKIND_SYSCALL_ENTRY:
 	  case TARGET_WAITKIND_SYSCALL_RETURN:
 	    /* Ignore gracefully during startup of the inferior.  */
-	    switch_to_thread (event_ptid);
+	    switch_to_thread (proc_target, event_ptid);
 	    break;
 
 	  case TARGET_WAITKIND_SIGNALLED:
@@ -527,12 +527,12 @@  startup_inferior (pid_t pid, int ntraps,
 	    /* Handle EXEC signals as if they were SIGTRAP signals.  */
 	    xfree (ws.value.execd_pathname);
 	    resume_signal = GDB_SIGNAL_TRAP;
-	    switch_to_thread (event_ptid);
+	    switch_to_thread (proc_target, event_ptid);
 	    break;
 
 	  case TARGET_WAITKIND_STOPPED:
 	    resume_signal = ws.value.sig;
-	    switch_to_thread (event_ptid);
+	    switch_to_thread (proc_target, event_ptid);
 	    break;
 	}
 
diff --git a/gdb/nat/fork-inferior.h b/gdb/nat/fork-inferior.h
index 1d0519fb26..269d60502b 100644
--- a/gdb/nat/fork-inferior.h
+++ b/gdb/nat/fork-inferior.h
@@ -22,6 +22,8 @@ 
 
 #include <string>
 
+struct process_stratum_target;
+
 /* Number of traps that happen between exec'ing the shell to run an
    inferior and when we finally get to the inferior code, not counting
    the exec for the shell.  This is 1 on all supported
@@ -50,7 +52,8 @@  extern pid_t fork_inferior (const char *exec_file_arg,
 /* Accept NTRAPS traps from the inferior.
 
    Return the ptid of the inferior being started.  */
-extern ptid_t startup_inferior (pid_t pid, int ntraps,
+extern ptid_t startup_inferior (process_stratum_target *proc_target,
+				pid_t pid, int ntraps,
 				struct target_waitstatus *mystatus,
 				ptid_t *myptid);
 
diff --git a/gdb/ppc-fbsd-tdep.c b/gdb/ppc-fbsd-tdep.c
index 290bd1fd88..a3efd860a6 100644
--- a/gdb/ppc-fbsd-tdep.c
+++ b/gdb/ppc-fbsd-tdep.c
@@ -35,6 +35,7 @@ 
 #include "ppc-fbsd-tdep.h"
 #include "fbsd-tdep.h"
 #include "solib-svr4.h"
+#include "inferior.h"
 
 
 /* 32-bit regset descriptions.  */
@@ -289,7 +290,8 @@  ppcfbsd_get_thread_local_address (struct gdbarch *gdbarch, ptid_t ptid,
   struct regcache *regcache;
   int tp_offset, tp_regnum;
 
-  regcache = get_thread_arch_regcache (ptid, gdbarch);
+  regcache = get_thread_arch_regcache (current_inferior ()->process_target (),
+				       ptid, gdbarch);
 
   if (tdep->wordsize == 4)
     {
diff --git a/gdb/proc-service.c b/gdb/proc-service.c
index b0741318ad..954d150a5e 100644
--- a/gdb/proc-service.c
+++ b/gdb/proc-service.c
@@ -70,17 +70,22 @@  static ps_err_e
 ps_xfer_memory (const struct ps_prochandle *ph, psaddr_t addr,
 		gdb_byte *buf, size_t len, int write)
 {
-  scoped_restore save_inferior_ptid = make_scoped_restore (&inferior_ptid);
-  int ret;
-  CORE_ADDR core_addr = ps_addr_to_core_addr (addr);
+  scoped_restore_current_inferior restore_inferior;
+  set_current_inferior (ph->thread->inf);
 
+  scoped_restore_current_program_space restore_current_progspace;
+  set_current_program_space (ph->thread->inf->pspace);
+
+  scoped_restore save_inferior_ptid = make_scoped_restore (&inferior_ptid);
   inferior_ptid = ph->thread->ptid;
 
+  CORE_ADDR core_addr = ps_addr_to_core_addr (addr);
+
+  int ret;
   if (write)
     ret = target_write_memory (core_addr, buf, len);
   else
     ret = target_read_memory (core_addr, buf, len);
-
   return (ret == 0 ? PS_OK : PS_ERR);
 }
 
@@ -138,7 +143,9 @@  static struct regcache *
 get_ps_regcache (struct ps_prochandle *ph, lwpid_t lwpid)
 {
   inferior *inf = ph->thread->inf;
-  return get_thread_arch_regcache (ptid_t (inf->pid, lwpid), inf->gdbarch);
+  return get_thread_arch_regcache (inf->process_target (),
+				   ptid_t (inf->pid, lwpid),
+				   inf->gdbarch);
 }
 
 /* Get the general registers of LWP LWPID within the target process PH
diff --git a/gdb/process-stratum-target.c b/gdb/process-stratum-target.c
index 1517088f0f..234a31378b 100644
--- a/gdb/process-stratum-target.c
+++ b/gdb/process-stratum-target.c
@@ -29,7 +29,7 @@  struct address_space *
 process_stratum_target::thread_address_space (ptid_t ptid)
 {
   /* Fall-back to the "main" address space of the inferior.  */
-  inferior *inf = find_inferior_ptid (ptid);
+  inferior *inf = find_inferior_ptid (this, ptid);
 
   if (inf == NULL || inf->aspace == NULL)
     internal_error (__FILE__, __LINE__,
@@ -43,7 +43,7 @@  process_stratum_target::thread_address_space (ptid_t ptid)
 struct gdbarch *
 process_stratum_target::thread_architecture (ptid_t ptid)
 {
-  inferior *inf = find_inferior_ptid (ptid);
+  inferior *inf = find_inferior_ptid (this, ptid);
   gdb_assert (inf != NULL);
   return inf->gdbarch;
 }
diff --git a/gdb/process-stratum-target.h b/gdb/process-stratum-target.h
index a14e4f3a72..6081c0a927 100644
--- a/gdb/process-stratum-target.h
+++ b/gdb/process-stratum-target.h
@@ -51,6 +51,22 @@  public:
   bool has_stack () override;
   bool has_registers () override;
   bool has_execution (inferior *inf) override;
+
+  /* True if any thread is, or may be executing.  We need to track
+     this separately because until we fully sync the thread list, we
+     won't know whether the target is fully stopped, even if we see
+     stop events for all known threads, because any of those threads
+     may have spawned new threads we haven't heard of yet.  */
+  bool threads_executing = false;
 };
 
+/* Downcast TARGET to process_stratum_target.  */
+
+static inline process_stratum_target *
+as_process_stratum_target (target_ops *target)
+{
+  gdb_assert (target->stratum () == process_stratum);
+  return static_cast<process_stratum_target *> (target);
+}
+
 #endif /* !defined (PROCESS_STRATUM_TARGET_H) */
diff --git a/gdb/python/py-threadevent.c b/gdb/python/py-threadevent.c
index c2af2fa3c1..9b2afc4802 100644
--- a/gdb/python/py-threadevent.c
+++ b/gdb/python/py-threadevent.c
@@ -27,7 +27,9 @@  py_get_event_thread (ptid_t ptid)
 {
   if (non_stop)
     {
-      thread_info *thread = find_thread_ptid (ptid);
+      thread_info *thread
+	= find_thread_ptid (current_inferior ()->process_target (),
+			    ptid);
       if (thread != nullptr)
 	return thread_to_thread_object (thread);
       PyErr_SetString (PyExc_RuntimeError, "Could not find event thread");
diff --git a/gdb/ravenscar-thread.c b/gdb/ravenscar-thread.c
index f217d949f6..095504ecbf 100644
--- a/gdb/ravenscar-thread.c
+++ b/gdb/ravenscar-thread.c
@@ -219,6 +219,9 @@  get_base_thread_from_ravenscar_task (ptid_t ptid)
 void
 ravenscar_thread_target::update_inferior_ptid ()
 {
+  process_stratum_target *proc_target
+    = as_process_stratum_target (this->beneath ());
+
   int base_cpu;
 
   m_base_ptid = inferior_ptid;
@@ -239,8 +242,8 @@  ravenscar_thread_target::update_inferior_ptid ()
   /* The running thread may not have been added to
      system.tasking.debug's list yet; so ravenscar_update_thread_list
      may not always add it to the thread list.  Add it here.  */
-  if (!find_thread_ptid (inferior_ptid))
-    add_thread (inferior_ptid);
+  if (!find_thread_ptid (proc_target, inferior_ptid))
+    add_thread (proc_target, inferior_ptid);
 }
 
 /* The Ravenscar Runtime exports a symbol which contains the ID of
@@ -336,12 +339,14 @@  ravenscar_thread_target::wait (ptid_t ptid,
 			       struct target_waitstatus *status,
 			       int options)
 {
+  process_stratum_target *beneath
+    = as_process_stratum_target (this->beneath ());
   ptid_t event_ptid;
 
   inferior_ptid = m_base_ptid;
   if (ptid != minus_one_ptid)
     ptid = m_base_ptid;
-  event_ptid = beneath ()->wait (ptid, status, 0);
+  event_ptid = beneath->wait (ptid, status, 0);
   /* Find any new threads that might have been created, and update
      inferior_ptid to the active thread.
 
@@ -367,8 +372,9 @@  ravenscar_thread_target::wait (ptid_t ptid,
 static void
 ravenscar_add_thread (struct ada_task_info *task)
 {
-  if (find_thread_ptid (task->ptid) == NULL)
-    add_thread (task->ptid);
+  process_stratum_target *targ = current_inferior ()->process_target ();
+  if (find_thread_ptid (targ, task->ptid) == NULL)
+    add_thread (targ, task->ptid);
 }
 
 void
diff --git a/gdb/record-btrace.c b/gdb/record-btrace.c
index 65d6a254ff..e31306e111 100644
--- a/gdb/record-btrace.c
+++ b/gdb/record-btrace.c
@@ -556,10 +556,11 @@  record_btrace_target::info_record ()
 
   DEBUG ("info");
 
-  tp = find_thread_ptid (inferior_ptid);
-  if (tp == NULL)
+  if (inferior_ptid == null_ptid)
     error (_("No thread."));
 
+  tp = inferior_thread ();
+
   validate_registers_access ();
 
   btinfo = &tp->btrace;
@@ -1373,7 +1374,8 @@  record_btrace_target::call_history_from (ULONGEST from, int size,
 enum record_method
 record_btrace_target::record_method (ptid_t ptid)
 {
-  struct thread_info * const tp = find_thread_ptid (ptid);
+  process_stratum_target *proc_target = current_inferior ()->process_target ();
+  thread_info *const tp = find_thread_ptid (proc_target, ptid);
 
   if (tp == NULL)
     error (_("No thread."));
@@ -1389,7 +1391,8 @@  record_btrace_target::record_method (ptid_t ptid)
 bool
 record_btrace_target::record_is_replaying (ptid_t ptid)
 {
-  for (thread_info *tp : all_non_exited_threads (ptid))
+  process_stratum_target *proc_target = current_inferior ()->process_target ();
+  for (thread_info *tp : all_non_exited_threads (proc_target, ptid))
     if (btrace_is_replaying (tp))
       return true;
 
@@ -1520,13 +1523,10 @@  record_btrace_target::remove_breakpoint (struct gdbarch *gdbarch,
 void
 record_btrace_target::fetch_registers (struct regcache *regcache, int regno)
 {
-  struct btrace_insn_iterator *replay;
-  struct thread_info *tp;
-
-  tp = find_thread_ptid (regcache->ptid ());
+  thread_info *tp = find_thread_ptid (regcache->target (), regcache->ptid ());
   gdb_assert (tp != NULL);
 
-  replay = tp->btrace.replay;
+  btrace_insn_iterator *replay = tp->btrace.replay;
   if (replay != NULL && !record_btrace_generating_corefile)
     {
       const struct btrace_insn *insn;
@@ -1967,6 +1967,8 @@  get_thread_current_frame_id (struct thread_info *tp)
 
   switch_to_thread (tp);
 
+  process_stratum_target *proc_target = tp->inf->process_target ();
+
   /* Clear the executing flag to allow changes to the current frame.
      We are not actually running, yet.  We just started a reverse execution
      command or a record goto command.
@@ -1975,7 +1977,7 @@  get_thread_current_frame_id (struct thread_info *tp)
      move the thread.  Since we need to recompute the stack, we temporarily
      set EXECUTING to flase.  */
   executing = tp->executing;
-  set_executing (inferior_ptid, false);
+  set_executing (proc_target, inferior_ptid, false);
 
   id = null_frame_id;
   try
@@ -1985,13 +1987,13 @@  get_thread_current_frame_id (struct thread_info *tp)
   catch (const gdb_exception &except)
     {
       /* Restore the previous execution state.  */
-      set_executing (inferior_ptid, executing);
+      set_executing (proc_target, inferior_ptid, executing);
 
       throw;
     }
 
   /* Restore the previous execution state.  */
-  set_executing (inferior_ptid, executing);
+  set_executing (proc_target, inferior_ptid, executing);
 
   return id;
 }
@@ -2154,12 +2156,15 @@  record_btrace_target::resume (ptid_t ptid, int step, enum gdb_signal signal)
   /* We just indicate the resume intent here.  The actual stepping happens in
      record_btrace_wait below.
 
-     For all-stop targets, we only step INFERIOR_PTID and continue others.  */
+      For all-stop targets, we only step INFERIOR_PTID and continue others.  */
+
+  process_stratum_target *proc_target = current_inferior ()->process_target ();
+
   if (!target_is_non_stop_p ())
     {
       gdb_assert (inferior_ptid.matches (ptid));
 
-      for (thread_info *tp : all_non_exited_threads (ptid))
+      for (thread_info *tp : all_non_exited_threads (proc_target, ptid))
 	{
 	  if (tp->ptid.matches (inferior_ptid))
 	    record_btrace_resume_thread (tp, flag);
@@ -2169,7 +2174,7 @@  record_btrace_target::resume (ptid_t ptid, int step, enum gdb_signal signal)
     }
   else
     {
-      for (thread_info *tp : all_non_exited_threads (ptid))
+      for (thread_info *tp : all_non_exited_threads (proc_target, ptid))
 	record_btrace_resume_thread (tp, flag);
     }
 
@@ -2527,7 +2532,8 @@  record_btrace_target::wait (ptid_t ptid, struct target_waitstatus *status,
     }
 
   /* Keep a work list of moving threads.  */
-  for (thread_info *tp : all_non_exited_threads (ptid))
+  process_stratum_target *proc_target = current_inferior ()->process_target ();
+  for (thread_info *tp : all_non_exited_threads (proc_target, ptid))
     if ((tp->btrace.flags & (BTHR_MOVE | BTHR_STOP)) != 0)
       moving.push_back (tp);
 
@@ -2647,7 +2653,10 @@  record_btrace_target::stop (ptid_t ptid)
     }
   else
     {
-      for (thread_info *tp : all_non_exited_threads (ptid))
+      process_stratum_target *proc_target
+	= current_inferior ()->process_target ();
+
+      for (thread_info *tp : all_non_exited_threads (proc_target, ptid))
 	{
 	  tp->btrace.flags &= ~BTHR_MOVE;
 	  tp->btrace.flags |= BTHR_STOP;
diff --git a/gdb/record-full.c b/gdb/record-full.c
index 1b03301da4..3737ee8492 100644
--- a/gdb/record-full.c
+++ b/gdb/record-full.c
@@ -1228,6 +1228,8 @@  record_full_wait_1 (struct target_ops *ops,
 		     interested in the event.  */
 
 		  registers_changed ();
+		  switch_to_thread (current_inferior ()->process_target (),
+				    ret);
 		  regcache = get_current_regcache ();
 		  tmp_pc = regcache_read_pc (regcache);
 		  const struct address_space *aspace = regcache->aspace ();
@@ -1260,14 +1262,17 @@  record_full_wait_1 (struct target_ops *ops,
 
                       if (gdbarch_software_single_step_p (gdbarch))
 			{
+			  process_stratum_target *proc_target
+			    = current_inferior ()->process_target ();
+
 			  /* Try to insert the software single step breakpoint.
 			     If insert success, set step to 0.  */
-			  set_executing (inferior_ptid, 0);
+			  set_executing (proc_target, inferior_ptid, 0);
 			  reinit_frame_cache ();
 
 			  step = !insert_single_step_breakpoints (gdbarch);
 
-			  set_executing (inferior_ptid, 1);
+			  set_executing (proc_target, inferior_ptid, 1);
 			}
 
 		      if (record_debug)
@@ -1290,6 +1295,8 @@  record_full_wait_1 (struct target_ops *ops,
     }
   else
     {
+      switch_to_thread (current_inferior ()->process_target (),
+			record_full_resume_ptid);
       struct regcache *regcache = get_current_regcache ();
       struct gdbarch *gdbarch = regcache->arch ();
       const struct address_space *aspace = regcache->aspace ();
diff --git a/gdb/regcache.c b/gdb/regcache.c
index 7184c74842..e0c0fd315e 100644
--- a/gdb/regcache.c
+++ b/gdb/regcache.c
@@ -196,10 +196,11 @@  reg_buffer::reg_buffer (gdbarch *gdbarch, bool has_pseudo)
     }
 }
 
-regcache::regcache (gdbarch *gdbarch, const address_space *aspace_)
+regcache::regcache (process_stratum_target *target, gdbarch *gdbarch,
+		    const address_space *aspace_)
 /* The register buffers.  A read/write register cache can only hold
    [0 .. gdbarch_num_regs).  */
-  : detached_regcache (gdbarch, false), m_aspace (aspace_)
+  : detached_regcache (gdbarch, false), m_aspace (aspace_), m_target (target)
 {
   m_ptid = minus_one_ptid;
 }
@@ -320,14 +321,19 @@  reg_buffer::assert_regnum (int regnum) const
 std::forward_list<regcache *> regcache::current_regcache;
 
 struct regcache *
-get_thread_arch_aspace_regcache (ptid_t ptid, struct gdbarch *gdbarch,
+get_thread_arch_aspace_regcache (process_stratum_target *target,
+				 ptid_t ptid, struct gdbarch *gdbarch,
 				 struct address_space *aspace)
 {
+  gdb_assert (target != nullptr);
+
   for (const auto &regcache : regcache::current_regcache)
-    if (regcache->ptid () == ptid && regcache->arch () == gdbarch)
+    if (regcache->target () == target
+	&& regcache->ptid () == ptid
+	&& regcache->arch () == gdbarch)
       return regcache;
 
-  regcache *new_regcache = new regcache (gdbarch, aspace);
+  regcache *new_regcache = new regcache (target, gdbarch, aspace);
 
   regcache::current_regcache.push_front (new_regcache);
   new_regcache->set_ptid (ptid);
@@ -336,26 +342,38 @@  get_thread_arch_aspace_regcache (ptid_t ptid, struct gdbarch *gdbarch,
 }
 
 struct regcache *
-get_thread_arch_regcache (ptid_t ptid, struct gdbarch *gdbarch)
+get_thread_arch_regcache (process_stratum_target *target, ptid_t ptid,
+			  struct gdbarch *gdbarch)
 {
+  scoped_restore_current_inferior restore_current_inferior;
+  set_current_inferior (find_inferior_ptid (target, ptid));
   address_space *aspace = target_thread_address_space (ptid);
 
-  return get_thread_arch_aspace_regcache  (ptid, gdbarch, aspace);
+  return get_thread_arch_aspace_regcache (target, ptid, gdbarch, aspace);
 }
 
+static process_stratum_target *current_thread_target;
 static ptid_t current_thread_ptid;
 static struct gdbarch *current_thread_arch;
 
 struct regcache *
-get_thread_regcache (ptid_t ptid)
+get_thread_regcache (process_stratum_target *target, ptid_t ptid)
 {
-  if (!current_thread_arch || current_thread_ptid != ptid)
+  if (!current_thread_arch
+      || target != current_thread_target
+      || current_thread_ptid != ptid)
     {
+      gdb_assert (ptid != null_ptid);
+
       current_thread_ptid = ptid;
+      current_thread_target = target;
+
+      scoped_restore_current_inferior restore_current_inferior;
+      set_current_inferior (find_inferior_ptid (target, ptid));
       current_thread_arch = target_thread_architecture (ptid);
     }
 
-  return get_thread_arch_regcache (ptid, current_thread_arch);
+  return get_thread_arch_regcache (target, ptid, current_thread_arch);
 }
 
 /* See regcache.h.  */
@@ -363,7 +381,8 @@  get_thread_regcache (ptid_t ptid)
 struct regcache *
 get_thread_regcache (thread_info *thread)
 {
-  return get_thread_regcache (thread->ptid);
+  return get_thread_regcache (thread->inf->process_target (),
+			      thread->ptid);
 }
 
 struct regcache *
@@ -377,7 +396,11 @@  get_current_regcache (void)
 struct regcache *
 get_thread_regcache_for_ptid (ptid_t ptid)
 {
-  return get_thread_regcache (ptid);
+  /* This function doesn't take a process_stratum_target parameter
+     because it's a common/ routine implemented by both gdb and
+     gdbserver.  It always refers to a ptid of the current target. */
+  process_stratum_target *proc_target = current_inferior ()->process_target ();
+  return get_thread_regcache (proc_target, ptid);
 }
 
 /* Observer for the target_changed event.  */
@@ -412,29 +435,34 @@  regcache::regcache_thread_ptid_changed (ptid_t old_ptid, ptid_t new_ptid)
    Indicate that registers may have changed, so invalidate the cache.  */
 
 void
-registers_changed_ptid (ptid_t ptid)
+registers_changed_ptid (process_stratum_target *target, ptid_t ptid)
 {
   for (auto oit = regcache::current_regcache.before_begin (),
 	 it = std::next (oit);
        it != regcache::current_regcache.end ();
        )
     {
-      if ((*it)->ptid ().matches (ptid))
+      struct regcache *regcache = *it;
+      if ((target == nullptr || regcache->target () == target)
+	  && regcache->ptid ().matches (ptid))
 	{
-	  delete *it;
+	  delete regcache;
 	  it = regcache::current_regcache.erase_after (oit);
 	}
       else
 	oit = it++;
     }
 
-  if (current_thread_ptid.matches (ptid))
+  if ((target == nullptr || current_thread_target == target)
+      && current_thread_ptid.matches (ptid))
     {
+      current_thread_target = NULL;
       current_thread_ptid = null_ptid;
       current_thread_arch = NULL;
     }
 
-  if (inferior_ptid.matches (ptid))
+  if ((target == nullptr || current_inferior ()->process_target () == target)
+      && inferior_ptid.matches (ptid))
     {
       /* We just deleted the regcache of the current thread.  Need to
 	 forget about any frames we have cached, too.  */
@@ -447,13 +475,13 @@  registers_changed_ptid (ptid_t ptid)
 void
 registers_changed_thread (thread_info *thread)
 {
-  registers_changed_ptid (thread->ptid);
+  registers_changed_ptid (thread->inf->process_target (), thread->ptid);
 }
 
 void
 registers_changed (void)
 {
-  registers_changed_ptid (minus_one_ptid);
+  registers_changed_ptid (nullptr, minus_one_ptid);
 }
 
 void
@@ -1400,6 +1428,21 @@  public:
   }
 };
 
+/* Wrapper around get_thread_arch_aspace_regcache that does some self checks.  */
+
+static void
+test_get_thread_arch_aspace_regcache (process_stratum_target *target,
+				      ptid_t ptid, struct gdbarch *gdbarch,
+				      address_space *aspace)
+{
+  struct regcache *regcache
+    = get_thread_arch_aspace_regcache (target, ptid, gdbarch, aspace);
+  SELF_CHECK (regcache != NULL);
+  SELF_CHECK (regcache->target () == target);
+  SELF_CHECK (regcache->ptid () == ptid);
+  SELF_CHECK (regcache->aspace () == aspace);
+}
+
 static void
 current_regcache_test (void)
 {
@@ -1408,47 +1451,61 @@  current_regcache_test (void)
 
   ptid_t ptid1 (1), ptid2 (2), ptid3 (3);
 
-  /* Get regcache from ptid1, a new regcache is added to
-     current_regcache.  */
-  regcache *regcache = get_thread_arch_aspace_regcache (ptid1,
-							target_gdbarch (),
-							NULL);
+  test_target_ops test_target1;
+  test_target_ops test_target2;
 
-  SELF_CHECK (regcache != NULL);
-  SELF_CHECK (regcache->ptid () == ptid1);
+  /* Get regcache from (target1,ptid1), a new regcache is added to
+     current_regcache.  */
+  test_get_thread_arch_aspace_regcache (&test_target1, ptid1,
+					target_gdbarch (),
+					NULL);
   SELF_CHECK (regcache_access::current_regcache_size () == 1);
 
-  /* Get regcache from ptid2, a new regcache is added to
+  /* Get regcache from (target1,ptid2), a new regcache is added to
      current_regcache.  */
-  regcache = get_thread_arch_aspace_regcache (ptid2,
-					      target_gdbarch (),
-					      NULL);
-  SELF_CHECK (regcache != NULL);
-  SELF_CHECK (regcache->ptid () == ptid2);
+  test_get_thread_arch_aspace_regcache (&test_target1, ptid2,
+					target_gdbarch (),
+					NULL);
   SELF_CHECK (regcache_access::current_regcache_size () == 2);
 
-  /* Get regcache from ptid3, a new regcache is added to
+  /* Get regcache from (target1,ptid3), a new regcache is added to
      current_regcache.  */
-  regcache = get_thread_arch_aspace_regcache (ptid3,
-					      target_gdbarch (),
-					      NULL);
-  SELF_CHECK (regcache != NULL);
-  SELF_CHECK (regcache->ptid () == ptid3);
+  test_get_thread_arch_aspace_regcache (&test_target1, ptid3,
+					target_gdbarch (),
+					NULL);
   SELF_CHECK (regcache_access::current_regcache_size () == 3);
 
-  /* Get regcache from ptid2 again, nothing is added to
+  /* Get regcache from (target1,ptid2) again, nothing is added to
      current_regcache.  */
-  regcache = get_thread_arch_aspace_regcache (ptid2,
-					      target_gdbarch (),
-					      NULL);
-  SELF_CHECK (regcache != NULL);
-  SELF_CHECK (regcache->ptid () == ptid2);
+  test_get_thread_arch_aspace_regcache (&test_target1, ptid2,
+					target_gdbarch (),
+					NULL);
   SELF_CHECK (regcache_access::current_regcache_size () == 3);
 
-  /* Mark ptid2 is changed, so regcache of ptid2 should be removed from
-     current_regcache.  */
-  registers_changed_ptid (ptid2);
-  SELF_CHECK (regcache_access::current_regcache_size () == 2);
+  /* Get regcache from (target2,ptid2), a new regcache is added to
+     current_regcache, since this time we're using a differen
+     target.  */
+  test_get_thread_arch_aspace_regcache (&test_target2, ptid2,
+					target_gdbarch (),
+					NULL);
+  SELF_CHECK (regcache_access::current_regcache_size () == 4);
+
+  /* Mark that (target1,ptid2) changed.  The regcache of (target1,
+     ptid2) should be removed from current_regcache.  */
+  registers_changed_ptid (&test_target1, ptid2);
+  SELF_CHECK (regcache_access::current_regcache_size () == 3);
+
+  /* Get the regcache from (target2,ptid2) again, confirming the
+     registers_changed_ptid call above did not delete it.  */
+  test_get_thread_arch_aspace_regcache (&test_target2, ptid2,
+					target_gdbarch (),
+					NULL);
+  SELF_CHECK (regcache_access::current_regcache_size () == 3);
+
+  /* Confirm that marking all regcaches of all targets as changed
+     clears current_regcache.  */
+  registers_changed_ptid (nullptr, minus_one_ptid);
+  SELF_CHECK (regcache_access::current_regcache_size () == 0);
 }
 
 class target_ops_no_register : public test_target_ops
@@ -1509,8 +1566,9 @@  target_ops_no_register::xfer_partial (enum target_object object,
 class readwrite_regcache : public regcache
 {
 public:
-  readwrite_regcache (struct gdbarch *gdbarch)
-    : regcache (gdbarch, nullptr)
+  readwrite_regcache (process_stratum_target *target,
+		      struct gdbarch *gdbarch)
+    : regcache (target, gdbarch, nullptr)
   {}
 };
 
@@ -1576,7 +1634,7 @@  cooked_read_test (struct gdbarch *gdbarch)
 	break;
     }
 
-  readwrite_regcache readwrite (gdbarch);
+  readwrite_regcache readwrite (&mock_target, gdbarch);
   gdb::def_vector<gdb_byte> buf (register_size (gdbarch, nonzero_regnum));
 
   readwrite.raw_read (nonzero_regnum, buf.data ());
@@ -1712,7 +1770,7 @@  cooked_write_test (struct gdbarch *gdbarch)
     }
   } pop_targets;
 
-  readwrite_regcache readwrite (gdbarch);
+  readwrite_regcache readwrite (&mock_target, gdbarch);
 
   const int num_regs = gdbarch_num_cooked_regs (gdbarch);
 
diff --git a/gdb/regcache.h b/gdb/regcache.h
index f7d4bc60f6..0303a9334a 100644
--- a/gdb/regcache.h
+++ b/gdb/regcache.h
@@ -29,17 +29,20 @@  struct regset;
 struct gdbarch;
 struct address_space;
 class thread_info;
+struct process_stratum_target;
 
 extern struct regcache *get_current_regcache (void);
-extern struct regcache *get_thread_regcache (ptid_t ptid);
+extern struct regcache *get_thread_regcache (process_stratum_target *target,
+					     ptid_t ptid);
 
 /* Get the regcache of THREAD.  */
 extern struct regcache *get_thread_regcache (thread_info *thread);
 
-extern struct regcache *get_thread_arch_regcache (ptid_t, struct gdbarch *);
-extern struct regcache *get_thread_arch_aspace_regcache (ptid_t,
-							 struct gdbarch *,
-							 struct address_space *);
+extern struct regcache *get_thread_arch_regcache
+  (process_stratum_target *targ, ptid_t, struct gdbarch *);
+extern struct regcache *get_thread_arch_aspace_regcache
+  (process_stratum_target *target, ptid_t,
+   struct gdbarch *, struct address_space *);
 
 extern enum register_status
   regcache_raw_read_signed (struct regcache *regcache,
@@ -385,13 +388,19 @@  public:
     this->m_ptid = ptid;
   }
 
+  process_stratum_target *target () const
+  {
+    return m_target;
+  }
+
 /* Dump the contents of a register from the register cache to the target
    debug.  */
   void debug_print_register (const char *func, int regno);
 
   static void regcache_thread_ptid_changed (ptid_t old_ptid, ptid_t new_ptid);
 protected:
-  regcache (gdbarch *gdbarch, const address_space *aspace_);
+  regcache (process_stratum_target *target, gdbarch *gdbarch,
+	    const address_space *aspace);
 
   static std::forward_list<regcache *> current_regcache;
 
@@ -421,14 +430,16 @@  private:
 
   /* If this is a read-write cache, which thread's registers is
      it connected to?  */
+  process_stratum_target *m_target;
   ptid_t m_ptid;
 
   friend struct regcache *
-  get_thread_arch_aspace_regcache (ptid_t ptid, struct gdbarch *gdbarch,
+  get_thread_arch_aspace_regcache (process_stratum_target *target, ptid_t ptid,
+				   struct gdbarch *gdbarch,
 				   struct address_space *aspace);
 
   friend void
-  registers_changed_ptid (ptid_t ptid);
+  registers_changed_ptid (process_stratum_target *target, ptid_t ptid);
 };
 
 class readonly_detached_regcache : public readable_regcache
@@ -451,7 +462,8 @@  public:
 };
 
 extern void registers_changed (void);
-extern void registers_changed_ptid (ptid_t);
+extern void registers_changed_ptid (process_stratum_target *target,
+				    ptid_t ptid);
 
 /* Indicate that registers of THREAD may have changed, so invalidate
    the cache.  */
diff --git a/gdb/remote.c b/gdb/remote.c
index ef6eb99999..168fff3047 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -277,6 +277,8 @@  public: /* data */
 
   /* The status of the stub support for the various vCont actions.  */
   vCont_action_support supports_vCont;
+  /* Whether vCont support was probed already.  */
+  bool supports_vCont_probed;
 
   /* True if the user has pressed Ctrl-C, but the target hasn't
      responded to that.  */
@@ -536,6 +538,8 @@  public:
 
   void async (int) override;
 
+  int async_wait_fd () override;
+
   void thread_events (int) override;
 
   int can_do_single_step () override;
@@ -1025,7 +1029,7 @@  static void remote_console_output (const char *msg);
 
 static void remote_btrace_reset (remote_state *rs);
 
-static void remote_unpush_and_throw (void);
+static void remote_unpush_and_throw (remote_target *target);
 
 /* For "remote".  */
 
@@ -1395,7 +1399,7 @@  remote_arch_state::remote_arch_state (struct gdbarch *gdbarch)
 static remote_target *
 get_current_remote_target ()
 {
-  target_ops *proc_target = find_target_at (process_stratum);
+  target_ops *proc_target = current_inferior ()->process_target ();
   return dynamic_cast<remote_target *> (proc_target);
 }
 
@@ -1676,6 +1680,7 @@  show_memory_packet_size (struct memory_packet_config *config)
     }
 }
 
+/* FIXME: needs to be per-remote-target.  */
 static struct memory_packet_config memory_write_packet_config =
 {
   "memory-write-packet-size",
@@ -1732,6 +1737,7 @@  remote_target::get_memory_write_packet_size ()
   return get_memory_packet_size (&memory_write_packet_config);
 }
 
+/* FIXME: needs to be per-remote-target.  */
 static struct memory_packet_config memory_read_packet_config =
 {
   "memory-read-packet-size",
@@ -2090,6 +2096,9 @@  enum {
   PACKET_MAX
 };
 
+/* FIXME: needs to be per-remote-target.  Ignoring this for now,
+   assuming all remote targets are the same server (thus all support
+   the same packets).  */
 static struct packet_config remote_protocol_packets[PACKET_MAX];
 
 /* Returns the packet's corresponding "set remote foo-packet" command
@@ -2391,6 +2400,7 @@  remote_target::remote_add_inferior (bool fake_pid_p, int pid, int attached,
 	  inf = add_inferior_with_spaces ();
 	}
       switch_to_inferior_no_thread (inf);
+      push_target (this);
       inferior_appeared (inf, pid);
     }
 
@@ -2406,7 +2416,8 @@  remote_target::remote_add_inferior (bool fake_pid_p, int pid, int attached,
 }
 
 static remote_thread_info *get_remote_thread_info (thread_info *thread);
-static remote_thread_info *get_remote_thread_info (ptid_t ptid);
+static remote_thread_info *get_remote_thread_info (remote_target *target,
+						   ptid_t ptid);
 
 /* Add thread PTID to GDB's thread list.  Tag it as executing/running
    according to RUNNING.  */
@@ -2424,13 +2435,13 @@  remote_target::remote_add_thread (ptid_t ptid, bool running, bool executing)
      might be confusing to the user.  Be silent then, preserving the
      age old behavior.  */
   if (rs->starting_up)
-    thread = add_thread_silent (ptid);
+    thread = add_thread_silent (this, ptid);
   else
-    thread = add_thread (ptid);
+    thread = add_thread (this, ptid);
 
   get_remote_thread_info (thread)->vcont_resumed = executing;
-  set_executing (ptid, executing);
-  set_running (ptid, running);
+  set_executing (this, ptid, executing);
+  set_running (this, ptid, running);
 
   return thread;
 }
@@ -2453,7 +2464,7 @@  remote_target::remote_notice_new_inferior (ptid_t currthread, int executing)
   /* If this is a new thread, add it to GDB's thread list.
      If we leave it up to WFI to do this, bad things will happen.  */
 
-  thread_info *tp = find_thread_ptid (currthread);
+  thread_info *tp = find_thread_ptid (this, currthread);
   if (tp != NULL && tp->state == THREAD_EXITED)
     {
       /* We're seeing an event on a thread id we knew had exited.
@@ -2462,7 +2473,7 @@  remote_target::remote_notice_new_inferior (ptid_t currthread, int executing)
       return;
     }
 
-  if (!in_thread_list (currthread))
+  if (!in_thread_list (this, currthread))
     {
       struct inferior *inf = NULL;
       int pid = currthread.pid ();
@@ -2475,8 +2486,8 @@  remote_target::remote_notice_new_inferior (ptid_t currthread, int executing)
 	     stub doesn't support qC.  This is the first stop reported
 	     after an attach, so this is the main thread.  Update the
 	     ptid in the thread list.  */
-	  if (in_thread_list (ptid_t (pid)))
-	    thread_change_ptid (inferior_ptid, currthread);
+	  if (in_thread_list (this, ptid_t (pid)))
+	    thread_change_ptid (this, inferior_ptid, currthread);
 	  else
 	    {
 	      remote_add_thread (currthread, running, executing);
@@ -2492,7 +2503,7 @@  remote_target::remote_notice_new_inferior (ptid_t currthread, int executing)
 	     doesn't support qC.  This is the first stop reported
 	     after an attach, so this is the main thread.  Update the
 	     ptid in the thread list.  */
-	  thread_change_ptid (inferior_ptid, currthread);
+	  thread_change_ptid (this, inferior_ptid, currthread);
 	  return;
 	}
 
@@ -2500,7 +2511,7 @@  remote_target::remote_notice_new_inferior (ptid_t currthread, int executing)
 	 extended-remote which already was debugging an inferior, we
 	 may not know about it yet.  Add it before adding its child
 	 thread, so notifications are emitted in a sensible order.  */
-      if (find_inferior_pid (currthread.pid ()) == NULL)
+      if (find_inferior_pid (this, currthread.pid ()) == NULL)
 	{
 	  struct remote_state *rs = get_remote_state ();
 	  bool fake_pid_p = !remote_multi_process_p (rs);
@@ -2540,10 +2551,12 @@  get_remote_thread_info (thread_info *thread)
   return static_cast<remote_thread_info *> (thread->priv.get ());
 }
 
+/* Return PTID's private thread data, creating it if necessary.  */
+
 static remote_thread_info *
-get_remote_thread_info (ptid_t ptid)
+get_remote_thread_info (remote_target *target, ptid_t ptid)
 {
-  thread_info *thr = find_thread_ptid (ptid);
+  thread_info *thr = find_thread_ptid (target, ptid);
   return get_remote_thread_info (thr);
 }
 
@@ -3803,6 +3816,9 @@  remote_target::update_thread_list ()
 	 target.  */
       for (thread_info *tp : all_threads_safe ())
 	{
+	  if (tp->inf->process_target () != this)
+	    continue;
+
 	  if (!context.contains_thread (tp->ptid))
 	    {
 	      /* Not found.  */
@@ -3828,7 +3844,7 @@  remote_target::update_thread_list ()
 
 	      remote_notice_new_inferior (item.ptid, executing);
 
-	      thread_info *tp = find_thread_ptid (item.ptid);
+	      thread_info *tp = find_thread_ptid (this, item.ptid);
 	      remote_thread_info *info = get_remote_thread_info (tp);
 	      info->core = item.core;
 	      info->extra = std::move (item.extra);
@@ -4030,13 +4046,6 @@  remote_target::close ()
   /* Make sure we leave stdin registered in the event loop.  */
   terminal_ours ();
 
-  /* We don't have a connection to the remote stub anymore.  Get rid
-     of all the inferiors and their threads we were controlling.
-     Reset inferior_ptid to null_ptid first, as otherwise has_stack_frame
-     will be unable to find the thread corresponding to (pid, 0, 0).  */
-  inferior_ptid = null_ptid;
-  discard_all_inferiors ();
-
   trace_reset_local_state ();
 
   delete this;
@@ -4346,7 +4355,7 @@  remote_target::add_current_inferior_and_thread (char *wait_status)
   /* Add the main thread and switch to it.  Don't try reading
      registers yet, since we haven't fetched the target description
      yet.  */
-  thread_info *tp = add_thread_silent (curr_ptid);
+  thread_info *tp = add_thread_silent (this, curr_ptid);
   switch_to_thread_no_regs (tp);
 }
 
@@ -4422,7 +4431,7 @@  remote_target::process_initial_stop_replies (int from_tty)
       if (ignore_event)
 	continue;
 
-      struct thread_info *evthread = find_thread_ptid (event_ptid);
+      thread_info *evthread = find_thread_ptid (this, event_ptid);
 
       if (ws.kind == TARGET_WAITKIND_STOPPED)
 	{
@@ -4442,14 +4451,14 @@  remote_target::process_initial_stop_replies (int from_tty)
 	  || ws.value.sig != GDB_SIGNAL_0)
 	evthread->suspend.waitstatus_pending_p = 1;
 
-      set_executing (event_ptid, 0);
-      set_running (event_ptid, 0);
+      set_executing (this, event_ptid, 0);
+      set_running (this, event_ptid, 0);
       get_remote_thread_info (evthread)->vcont_resumed = 0;
     }
 
   /* "Notice" the new inferiors before anything related to
      registers/memory.  */
-  for (inferior *inf : all_non_exited_inferiors ())
+  for (inferior *inf : all_non_exited_inferiors (this))
     {
       inf->needs_setup = 1;
 
@@ -4470,7 +4479,7 @@  remote_target::process_initial_stop_replies (int from_tty)
 
       /* If all threads of an inferior were already stopped, we
 	 haven't setup the inferior yet.  */
-      for (inferior *inf : all_non_exited_inferiors ())
+      for (inferior *inf : all_non_exited_inferiors (this))
 	{
 	  if (inf->needs_setup)
 	    {
@@ -4484,7 +4493,7 @@  remote_target::process_initial_stop_replies (int from_tty)
   /* Now go over all threads that are stopped, and print their current
      frame.  If all-stop, then if there's a signalled thread, pick
      that as current.  */
-  for (thread_info *thread : all_non_exited_threads ())
+  for (thread_info *thread : all_non_exited_threads (this))
     {
       if (first == NULL)
 	first = thread;
@@ -4523,7 +4532,7 @@  remote_target::process_initial_stop_replies (int from_tty)
   /* For "info program".  */
   thread_info *thread = inferior_thread ();
   if (thread->state == THREAD_STOPPED)
-    set_last_target_status (inferior_ptid, thread->suspend.waitstatus);
+    set_last_target_status (this, inferior_ptid, thread->suspend.waitstatus);
 }
 
 /* Start the remote connection and sync state.  */
@@ -4696,7 +4705,7 @@  remote_target::start_remote (int from_tty, int extended_p)
       /* Let the stub know that we want it to return the thread.  */
       set_continue_thread (minus_one_ptid);
 
-      if (thread_count () == 0)
+      if (thread_count (this) == 0)
 	{
 	  /* Target has no concept of threads at all.  GDB treats
 	     non-threaded target as single-threaded; add a main
@@ -4722,14 +4731,15 @@  remote_target::start_remote (int from_tty, int extended_p)
 		                    "warning: couldn't determine remote "
 				    "current thread; picking first in list.\n");
 
-	      for (thread_info *tp : all_non_exited_threads ())
+	      for (thread_info *tp : all_non_exited_threads (this,
+							     minus_one_ptid))
 		{
 		  switch_to_thread (tp);
 		  break;
 		}
 	    }
 	  else
-	    switch_to_thread (find_thread_ptid (curr_thread));
+	    switch_to_thread (find_thread_ptid (this, curr_thread));
 	}
 
       /* init_wait_for_inferior should be called before get_offsets in order
@@ -4788,7 +4798,7 @@  remote_target::start_remote (int from_tty, int extended_p)
 	  remote_notif_get_pending_events (notif);
 	}
 
-      if (thread_count () == 0)
+      if (thread_count (this) == 0)
 	{
 	  if (!extended_p)
 	    error (_("The target is not running (try extended-remote?)"));
@@ -5445,7 +5455,7 @@  remote_target::remote_serial_quit_handler ()
 	{
 	  if (query (_("The target is not responding to GDB commands.\n"
 		       "Stop debugging it? ")))
-	    remote_unpush_and_throw ();
+	    remote_unpush_and_throw (this);
 	}
       /* If ^C has already been sent once, offer to disconnect.  */
       else if (!target_terminal::is_ours () && rs->ctrlc_pending_p)
@@ -5469,19 +5479,29 @@  remote_serial_quit_handler ()
   curr_quit_handler_target->remote_serial_quit_handler ();
 }
 
-/* Remove any of the remote.c targets from target stack.  Upper targets depend
-   on it so remove them first.  */
+/* Remove the remote target from the target stack of each inferior
+   that is using it.  Upper targets depend on it so remove them
+   first.  */
 
 static void
-remote_unpush_target (void)
+remote_unpush_target (remote_target *target)
 {
-  pop_all_targets_at_and_above (process_stratum);
+  /* We have to unpush the target from all inferiors, even those that
+     aren't running.  */
+  scoped_restore_current_inferior restore_current_inferior;
+
+  for (inferior *inf : all_inferiors (target))
+    {
+      switch_to_inferior_no_thread (inf);
+      pop_all_targets_at_and_above (process_stratum);
+      generic_mourn_inferior ();
+    }
 }
 
 static void
-remote_unpush_and_throw (void)
+remote_unpush_and_throw (remote_target *target)
 {
-  remote_unpush_target ();
+  remote_unpush_target (target);
   throw_error (TARGET_CLOSE_ERROR, _("Disconnected from target."));
 }
 
@@ -5498,7 +5518,7 @@  remote_target::open_1 (const char *name, int from_tty, int extended_p)
   /* If we're connected to a running target, target_preopen will kill it.
      Ask this question first, before target_preopen has a chance to kill
      anything.  */
-  if (curr_remote != NULL && !have_inferiors ())
+  if (curr_remote != NULL && !target_has_execution)
     {
       if (from_tty
 	  && !query (_("Already connected to a remote target.  Disconnect? ")))
@@ -5627,7 +5647,7 @@  remote_target::open_1 (const char *name, int from_tty, int extended_p)
 	/* Pop the partially set up target - unless something else did
 	   already before throwing the exception.  */
 	if (ex.error != TARGET_CLOSE_ERROR)
-	  remote_unpush_target ();
+	  remote_unpush_target (remote);
 	throw;
       }
   }
@@ -5691,10 +5711,10 @@  remote_target::remote_detach_1 (inferior *inf, int from_tty)
   remote_detach_pid (pid);
 
   /* Exit only if this is the only active inferior.  */
-  if (from_tty && !rs->extended && number_of_live_inferiors () == 1)
+  if (from_tty && !rs->extended && number_of_live_inferiors (this) == 1)
     puts_filtered (_("Ending remote debugging.\n"));
 
-  struct thread_info *tp = find_thread_ptid (inferior_ptid);
+  thread_info *tp = find_thread_ptid (this, inferior_ptid);
 
   /* Check to see if we are detaching a fork parent.  Note that if we
      are detaching a fork child, tp == NULL.  */
@@ -5796,10 +5816,10 @@  remote_target::disconnect (const char *args, int from_tty)
     error (_("Argument given to \"disconnect\" when remotely debugging."));
 
   /* Make sure we unpush even the extended remote targets.  Calling
-     target_mourn_inferior won't unpush, and remote_mourn won't
-     unpush if there is more than one inferior left.  */
-  unpush_target (this);
-  generic_mourn_inferior ();
+     target_mourn_inferior won't unpush, and
+     remote_target::mourn_inferior won't unpush if there is more than
+     one inferior left.  */
+  remote_unpush_target (this);
 
   if (from_tty)
     puts_filtered ("Ending remote debugging.\n");
@@ -5887,10 +5907,10 @@  extended_remote_target::attach (const char *args, int from_tty)
       inferior_ptid = remote_current_thread (inferior_ptid);
 
       /* Add the main thread to the thread list.  */
-      thread_info *thr = add_thread_silent (inferior_ptid);
+      thread_info *thr = add_thread_silent (this, inferior_ptid);
       /* Don't consider the thread stopped until we've processed the
 	 saved stop reply.  */
-      set_executing (thr->ptid, true);
+      set_executing (this, thr->ptid, true);
     }
 
   /* Next, if the target can specify a description, read it.  We do
@@ -5992,6 +6012,7 @@  remote_target::remote_vcont_probe ()
     }
 
   packet_ok (rs->buf, &remote_protocol_packets[PACKET_vCont]);
+  rs->supports_vCont_probed = true;
 }
 
 /* Helper function for building "vCont" resumptions.  Write a
@@ -6029,10 +6050,10 @@  remote_target::append_resumption (char *p, char *endp,
 	{
 	  /* If we don't know about the target thread's tid, then
 	     we're resuming magic_null_ptid (see caller).  */
-	  tp = find_thread_ptid (magic_null_ptid);
+	  tp = find_thread_ptid (this, magic_null_ptid);
 	}
       else
-	tp = find_thread_ptid (ptid);
+	tp = find_thread_ptid (this, ptid);
       gdb_assert (tp != NULL);
 
       if (tp->control.may_range_step)
@@ -6095,7 +6116,7 @@  char *
 remote_target::append_pending_thread_resumptions (char *p, char *endp,
 						  ptid_t ptid)
 {
-  for (thread_info *thread : all_non_exited_threads (ptid))
+  for (thread_info *thread : all_non_exited_threads (this, ptid))
     if (inferior_ptid != thread->ptid
 	&& thread->suspend.stop_signal != GDB_SIGNAL_0)
       {
@@ -6128,7 +6149,7 @@  remote_target::remote_resume_with_hc (ptid_t ptid, int step,
   else
     set_continue_thread (ptid);
 
-  for (thread_info *thread : all_non_exited_threads ())
+  for (thread_info *thread : all_non_exited_threads (this))
     resume_clear_thread_private_info (thread);
 
   buf = rs->buf.data ();
@@ -6265,9 +6286,9 @@  remote_target::resume (ptid_t ptid, int step, enum gdb_signal siggnal)
       remote_thread_info *remote_thr;
 
       if (minus_one_ptid == ptid || ptid.is_pid ())
-	remote_thr = get_remote_thread_info (inferior_ptid);
+	remote_thr = get_remote_thread_info (this, inferior_ptid);
       else
-	remote_thr = get_remote_thread_info (ptid);
+	remote_thr = get_remote_thread_info (this, ptid);
 
       remote_thr->last_resume_step = step;
       remote_thr->last_resume_sig = siggnal;
@@ -6499,7 +6520,7 @@  remote_target::commit_resume ()
   may_global_wildcard_vcont = 1;
 
   /* And assume every process is individually wildcard-able too.  */
-  for (inferior *inf : all_non_exited_inferiors ())
+  for (inferior *inf : all_non_exited_inferiors (this))
     {
       remote_inferior *priv = get_remote_inferior (inf);
 
@@ -6510,7 +6531,7 @@  remote_target::commit_resume ()
      disable process and global wildcard resumes appropriately.  */
   check_pending_events_prevent_wildcard_vcont (&may_global_wildcard_vcont);
 
-  for (thread_info *tp : all_non_exited_threads ())
+  for (thread_info *tp : all_non_exited_threads (this))
     {
       /* If a thread of a process is not meant to be resumed, then we
 	 can't wildcard that process.  */
@@ -6539,7 +6560,7 @@  remote_target::commit_resume ()
   struct vcont_builder vcont_builder (this);
 
   /* Threads first.  */
-  for (thread_info *tp : all_non_exited_threads ())
+  for (thread_info *tp : all_non_exited_threads (this))
     {
       remote_thread_info *remote_thr = get_remote_thread_info (tp);
 
@@ -6568,7 +6589,7 @@  remote_target::commit_resume ()
      supposed to be resumed.  */
   any_process_wildcard = 0;
 
-  for (inferior *inf : all_non_exited_inferiors ())
+  for (inferior *inf : all_non_exited_inferiors (this))
     {
       if (get_remote_inferior (inf)->may_wildcard_vcont)
 	{
@@ -6589,7 +6610,7 @@  remote_target::commit_resume ()
 	}
       else
 	{
-	  for (inferior *inf : all_non_exited_inferiors ())
+	  for (inferior *inf : all_non_exited_inferiors (this))
 	    {
 	      if (get_remote_inferior (inf)->may_wildcard_vcont)
 		{
@@ -6601,6 +6622,8 @@  remote_target::commit_resume ()
     }
 
   vcont_builder.flush ();
+
+  target_async (1);
 }
 
 
@@ -6616,7 +6639,10 @@  remote_target::remote_stop_ns (ptid_t ptid)
   char *p = rs->buf.data ();
   char *endp = p + get_remote_packet_size ();
 
-  if (packet_support (PACKET_vCont) == PACKET_SUPPORT_UNKNOWN)
+  /* FIXME: This supports_vCont_probed check is a workaround until
+     packet_support is per-connection.  */
+  if (packet_support (PACKET_vCont) == PACKET_SUPPORT_UNKNOWN
+      || !rs->supports_vCont_probed)
     remote_vcont_probe ();
 
   if (!rs->supports_vCont.t)
@@ -6773,7 +6799,7 @@  remote_target::interrupt_query ()
       if (query (_("The target is not responding to interrupt requests.\n"
 		   "Stop debugging it? ")))
 	{
-	  remote_unpush_target ();
+	  remote_unpush_target (this);
 	  throw_error (TARGET_CLOSE_ERROR, _("Disconnected from target."));
 	}
     }
@@ -6980,7 +7006,7 @@  remote_target::remove_new_fork_children (threads_listing_context *context)
 
   /* For any threads stopped at a fork event, remove the corresponding
      fork child threads from the CONTEXT list.  */
-  for (thread_info *thread : all_non_exited_threads ())
+  for (thread_info *thread : all_non_exited_threads (this))
     {
       struct target_waitstatus *ws = thread_pending_fork_status (thread);
 
@@ -7022,7 +7048,7 @@  remote_target::check_pending_events_prevent_wildcard_vcont
 	  || event->ws.kind == TARGET_WAITKIND_VFORKED)
 	*may_global_wildcard = 0;
 
-      struct inferior *inf = find_inferior_ptid (event->ptid);
+      struct inferior *inf = find_inferior_ptid (this, event->ptid);
 
       /* This may be the first time we heard about this process.
 	 Regardless, we must not do a global wildcard resume, otherwise
@@ -7388,9 +7414,10 @@  Packet: '%s'\n"),
 
 		  if (rsa == NULL)
 		    {
-		      inferior *inf = (event->ptid == null_ptid
-				       ? NULL
-				       : find_inferior_ptid (event->ptid));
+		      inferior *inf
+			= (event->ptid == null_ptid
+			   ? NULL
+			   : find_inferior_ptid (this, event->ptid));
 		      /* If this is the first time we learn anything
 			 about this process, skip the registers
 			 included in this packet, since we don't yet
@@ -7650,7 +7677,7 @@  remote_target::process_stop_reply (struct stop_reply *stop_reply,
       if (!stop_reply->regcache.empty ())
 	{
 	  struct regcache *regcache
-	    = get_thread_arch_regcache (ptid, stop_reply->arch);
+	    = get_thread_arch_regcache (this, ptid, stop_reply->arch);
 
 	  for (cached_reg_t &reg : stop_reply->regcache)
 	    {
@@ -7662,7 +7689,7 @@  remote_target::process_stop_reply (struct stop_reply *stop_reply,
 	}
 
       remote_notice_new_inferior (ptid, 0);
-      remote_thread_info *remote_thr = get_remote_thread_info (ptid);
+      remote_thread_info *remote_thr = get_remote_thread_info (this, ptid);
       remote_thr->core = stop_reply->core;
       remote_thr->stop_reason = stop_reply->stop_reason;
       remote_thr->watch_data_address = stop_reply->watch_data_address;
@@ -7732,9 +7759,9 @@  remote_target::wait_ns (ptid_t ptid, struct target_waitstatus *status, int optio
 /* Return the first resumed thread.  */
 
 static ptid_t
-first_remote_resumed_thread ()
+first_remote_resumed_thread (remote_target *target)
 {
-  for (thread_info *tp : all_non_exited_threads (minus_one_ptid))
+  for (thread_info *tp : all_non_exited_threads (target, minus_one_ptid))
     if (tp->resumed)
       return tp->ptid;
   return null_ptid;
@@ -7876,7 +7903,7 @@  remote_target::wait_as (ptid_t ptid, target_waitstatus *status, int options)
       if (event_ptid != null_ptid)
 	record_currthread (rs, event_ptid);
       else
-	event_ptid = first_remote_resumed_thread ();
+	event_ptid = first_remote_resumed_thread (this);
     }
   else
     /* A process exit.  Invalidate our notion of current thread.  */
@@ -8993,11 +9020,11 @@  remote_target::files_info ()
    for output compatibility with throw_perror_with_name.  */
 
 static void
-unpush_and_perror (const char *string)
+unpush_and_perror (remote_target *target, const char *string)
 {
   int saved_errno = errno;
 
-  remote_unpush_target ();
+  remote_unpush_target (target);
   throw_error (TARGET_CLOSE_ERROR, "%s: %s.", string,
 	       safe_strerror (saved_errno));
 }
@@ -9033,12 +9060,12 @@  remote_target::readchar (int timeout)
   switch ((enum serial_rc) ch)
     {
     case SERIAL_EOF:
-      remote_unpush_target ();
+      remote_unpush_target (this);
       throw_error (TARGET_CLOSE_ERROR, _("Remote connection closed"));
       /* no return */
     case SERIAL_ERROR:
-      unpush_and_perror (_("Remote communication error.  "
-			   "Target disconnected."));
+      unpush_and_perror (this, _("Remote communication error.  "
+				 "Target disconnected."));
       /* no return */
     case SERIAL_TIMEOUT:
       break;
@@ -9066,8 +9093,8 @@  remote_target::remote_serial_write (const char *str, int len)
 
   if (serial_write (rs->remote_desc, str, len))
     {
-      unpush_and_perror (_("Remote communication error.  "
-			   "Target disconnected."));
+      unpush_and_perror (this, _("Remote communication error.  "
+				 "Target disconnected."));
     }
 
   if (rs->got_ctrlc_during_io)
@@ -9584,7 +9611,7 @@  remote_target::getpkt_or_notif_sane_1 (gdb::char_vector *buf,
 
 	      if (forever)	/* Watchdog went off?  Kill the target.  */
 		{
-		  remote_unpush_target ();
+		  remote_unpush_target (this);
 		  throw_error (TARGET_CLOSE_ERROR,
 			       _("Watchdog timeout has expired.  "
 				 "Target detached."));
@@ -9694,7 +9721,7 @@  remote_target::kill_new_fork_children (int pid)
 
   /* Kill the fork child threads of any threads in process PID
      that are stopped at a fork event.  */
-  for (thread_info *thread : all_non_exited_threads ())
+  for (thread_info *thread : all_non_exited_threads (this))
     {
       struct target_waitstatus *ws = &thread->pending_follow;
 
@@ -9754,7 +9781,7 @@  remote_target::kill ()
      inferior, then we will tell gdbserver to exit and unpush the
      target.  */
   if (res == -1 && !remote_multi_process_p (rs)
-      && number_of_live_inferiors () == 1)
+      && number_of_live_inferiors (this) == 1)
     {
       remote_kill_k ();
 
@@ -9840,12 +9867,9 @@  remote_target::mourn_inferior ()
   discard_pending_stop_replies (current_inferior ());
 
   /* In 'target remote' mode with one inferior, we close the connection.  */
-  if (!rs->extended && number_of_live_inferiors () <= 1)
+  if (!rs->extended && number_of_live_inferiors (this) <= 1)
     {
-      unpush_target (this);
-
-      /* remote_close takes care of doing most of the clean up.  */
-      generic_mourn_inferior ();
+      remote_unpush_target (this);
       return;
     }
 
@@ -13444,7 +13468,7 @@  remote_target::set_disconnected_tracing (int val)
 int
 remote_target::core_of_thread (ptid_t ptid)
 {
-  struct thread_info *info = find_thread_ptid (ptid);
+  thread_info *info = find_thread_ptid (this, ptid);
 
   if (info != NULL && info->priv != NULL)
     return get_remote_thread_info (info)->core;
@@ -13724,7 +13748,7 @@  remote_target::remote_btrace_maybe_reopen ()
 
   scoped_restore_current_thread restore_thread;
 
-  for (thread_info *tp : all_non_exited_threads ())
+  for (thread_info *tp : all_non_exited_threads (this))
     {
       set_general_thread (tp->ptid);
 
@@ -13940,13 +13964,12 @@  char *
 remote_target::pid_to_exec_file (int pid)
 {
   static gdb::optional<gdb::char_vector> filename;
-  struct inferior *inf;
   char *annex = NULL;
 
   if (packet_support (PACKET_qXfer_exec_file) != PACKET_ENABLE)
     return NULL;
 
-  inf = find_inferior_pid (pid);
+  inferior *inf = find_inferior_pid (this, pid);
   if (inf == NULL)
     internal_error (__FILE__, __LINE__,
 		    _("not currently attached to process %d"), pid);
@@ -14007,7 +14030,7 @@  remote_target::thread_handle_to_thread_info (const gdb_byte *thread_handle,
 					     int handle_len,
 					     inferior *inf)
 {
-  for (thread_info *tp : all_non_exited_threads ())
+  for (thread_info *tp : all_non_exited_threads (this))
     {
       remote_thread_info *priv = get_remote_thread_info (tp);
 
@@ -14079,6 +14102,13 @@  remote_async_inferior_event_handler (gdb_client_data data)
   inferior_event_handler (INF_REG_EVENT, data);
 }
 
+int
+remote_target::async_wait_fd ()
+{
+  struct remote_state *rs = get_remote_state ();
+  return rs->remote_desc->fd;
+}
+
 void
 remote_target::async (int enable)
 {
diff --git a/gdb/riscv-fbsd-tdep.c b/gdb/riscv-fbsd-tdep.c
index a17d55b08b..926fd6b6e6 100644
--- a/gdb/riscv-fbsd-tdep.c
+++ b/gdb/riscv-fbsd-tdep.c
@@ -26,6 +26,7 @@ 
 #include "trad-frame.h"
 #include "tramp-frame.h"
 #include "gdbarch.h"
+#include "inferior.h"
 
 /* Register maps.  */
 
@@ -183,7 +184,8 @@  riscv_fbsd_get_thread_local_address (struct gdbarch *gdbarch, ptid_t ptid,
 {
   struct regcache *regcache;
 
-  regcache = get_thread_arch_regcache (ptid, gdbarch);
+  regcache = get_thread_arch_regcache (current_inferior ()->process_target (),
+				       ptid, gdbarch);
 
   target_fetch_registers (regcache, RISCV_TP_REGNUM);
 
diff --git a/gdb/sol2-tdep.c b/gdb/sol2-tdep.c
index 014b7d79a7..dd6f06ec95 100644
--- a/gdb/sol2-tdep.c
+++ b/gdb/sol2-tdep.c
@@ -58,7 +58,7 @@  sol2_core_pid_to_str (struct gdbarch *gdbarch, ptid_t ptid)
   /* GDB didn't use to put a NT_PSTATUS note in Solaris cores.  If
      that's missing, then we're dealing with a fake PID corelow.c made
      up.  */
-  inf = find_inferior_ptid (ptid);
+  inf = find_inferior_ptid (current_inferior ()->process_target (), ptid);
   if (inf == NULL || inf->fake_pid_p)
     return "<core>";
 
diff --git a/gdb/solib-spu.c b/gdb/solib-spu.c
index 5b97b9bcf6..71942fd937 100644
--- a/gdb/solib-spu.c
+++ b/gdb/solib-spu.c
@@ -88,7 +88,8 @@  spu_skip_standalone_loader (void)
 
       target_resume (inferior_ptid, 1, GDB_SIGNAL_0);
       target_wait (minus_one_ptid, &ws, 0);
-      set_executing (minus_one_ptid, 0);
+      set_executing (current_inferior ()->process_target (),
+		     minus_one_ptid, 0);
 
       inferior_thread ()->control.in_infcall = 0;
     }
diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
index ffae26bfc5..dc182f2480 100644
--- a/gdb/solib-svr4.c
+++ b/gdb/solib-svr4.c
@@ -2373,7 +2373,8 @@  enable_break (struct svr4_info *info, int from_tty)
       if (!load_addr_found)
 	{
 	  struct regcache *regcache
-	    = get_thread_arch_regcache (inferior_ptid, target_gdbarch ());
+	    = get_thread_arch_regcache (current_inferior ()->process_target (),
+					inferior_ptid, target_gdbarch ());
 
 	  load_addr = (regcache_read_pc (regcache)
 		       - exec_entry_point (tmp_bfd.get (), tmp_bfd_target));
diff --git a/gdb/spu-multiarch.c b/gdb/spu-multiarch.c
index 88ad291dbd..c1b2afd808 100644
--- a/gdb/spu-multiarch.c
+++ b/gdb/spu-multiarch.c
@@ -100,13 +100,15 @@  parse_spufs_run (ptid_t ptid, int *fd, CORE_ADDR *addr)
   if (gdbarch_bfd_arch_info (target_gdbarch ())->arch != bfd_arch_powerpc)
     return 0;
 
+  process_stratum_target *proc_target = current_inferior ()->process_target ();
+
   /* If we're called too early (e.g. after fork), we cannot
      access the inferior yet.  */
-  if (find_inferior_ptid (ptid) == NULL)
+  if (find_inferior_ptid (proc_target, ptid) == NULL)
     return 0;
 
   /* Get PPU-side registers.  */
-  regcache = get_thread_arch_regcache (ptid, target_gdbarch ());
+  regcache = get_thread_arch_regcache (proc_target, ptid, target_gdbarch ());
   tdep = gdbarch_tdep (target_gdbarch ());
 
   /* Fetch instruction preceding current NIP.  */
diff --git a/gdb/spu-tdep.c b/gdb/spu-tdep.c
index ddaee54b30..48d87cbcfa 100644
--- a/gdb/spu-tdep.c
+++ b/gdb/spu-tdep.c
@@ -1273,8 +1273,12 @@  spu2ppu_sniffer (const struct frame_unwind *self,
 	}
       else
 	{
-	  struct regcache *regcache;
-	  regcache = get_thread_arch_regcache (inferior_ptid, target_gdbarch ());
+	  process_stratum_target *proc_target
+	    = current_inferior ()->process_target ();
+	  struct regcache *regcache
+	    = get_thread_arch_regcache (proc_target, inferior_ptid,
+					target_gdbarch ());
+
 	  cache->regcache = new readonly_detached_regcache (*regcache);
 	  *this_prologue_cache = cache;
 	  return 1;
diff --git a/gdb/target-delegates.c b/gdb/target-delegates.c
index 52034fe436..5c2142b63f 100644
--- a/gdb/target-delegates.c
+++ b/gdb/target-delegates.c
@@ -83,6 +83,7 @@  struct dummy_target : public target_ops
   bool can_async_p () override;
   bool is_async_p () override;
   void async (int arg0) override;
+  int async_wait_fd () override;
   void thread_events (int arg0) override;
   bool supports_non_stop () override;
   bool always_non_stop_p () override;
@@ -251,6 +252,7 @@  struct debug_target : public target_ops
   bool can_async_p () override;
   bool is_async_p () override;
   void async (int arg0) override;
+  int async_wait_fd () override;
   void thread_events (int arg0) override;
   bool supports_non_stop () override;
   bool always_non_stop_p () override;
@@ -2162,6 +2164,31 @@  debug_target::async (int arg0)
   fputs_unfiltered (")\n", gdb_stdlog);
 }
 
+int
+target_ops::async_wait_fd ()
+{
+  return this->beneath ()->async_wait_fd ();
+}
+
+int
+dummy_target::async_wait_fd ()
+{
+  noprocess ();
+}
+
+int
+debug_target::async_wait_fd ()
+{
+  int result;
+  fprintf_unfiltered (gdb_stdlog, "-> %s->async_wait_fd (...)\n", this->beneath ()->shortname ());
+  result = this->beneath ()->async_wait_fd ();
+  fprintf_unfiltered (gdb_stdlog, "<- %s->async_wait_fd (", this->beneath ()->shortname ());
+  fputs_unfiltered (") = ", gdb_stdlog);
+  target_debug_print_int (result);
+  fputs_unfiltered ("\n", gdb_stdlog);
+  return result;
+}
+
 void
 target_ops::thread_events (int arg0)
 {
diff --git a/gdb/target.c b/gdb/target.c
index 68f874bf4b..541b95bc10 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -110,10 +110,6 @@  static std::unordered_map<const target_info *, target_open_ftype *>
 
 static struct target_ops *the_debug_target;
 
-/* The target stack.  */
-
-static target_stack g_target_stack;
-
 /* Top of target stack.  */
 /* The target structure we are currently using to talk to a process
    or file or whatever "inferior" we have.  */
@@ -121,7 +117,7 @@  static target_stack g_target_stack;
 target_ops *
 current_top_target ()
 {
-  return g_target_stack.top ();
+  return current_inferior ()->top_target ();
 }
 
 /* Command list for target.  */
@@ -226,7 +222,9 @@  target_has_registers_1 (void)
 bool
 target_has_execution_1 (inferior *inf)
 {
-  for (target_ops *t = current_top_target (); t != NULL; t = t->beneath ())
+  for (target_ops *t = inf->top_target ();
+       t != nullptr;
+       t = inf->find_target_beneath (t))
     if (t->has_execution (inf))
       return true;
 
@@ -501,13 +499,16 @@  target_terminal::info (const char *arg, int from_tty)
 bool
 target_supports_terminal_ours (void)
 {
-  /* This can be called before there is any target, so we must check
-     for nullptr here.  */
-  target_ops *top = current_top_target ();
+  /* The current top target is the target at the top of the target
+     stack of the current inferior.  While normally there's always an
+     inferior, we must check for nullptr here because we can get here
+     very early during startup, before the initial inferior is first
+     created.  */
+  inferior *inf = current_inferior ();
 
-  if (top == nullptr)
+  if (inf == nullptr)
     return false;
-  return top->supports_terminal_ours ();
+  return inf->top_target ()->supports_terminal_ours ();
 }
 
 static void
@@ -553,19 +554,32 @@  default_execution_direction (struct target_ops *self)
 to_execution_direction must be implemented for reverse async");
 }
 
+/* Decref a target and close if, if there are no references left.  */
+
+static void
+decref_target (target_ops *t)
+{
+  t->decref ();
+  if (t->refcount () == 0)
+    target_close (t);
+}
+
 /* See target.h.  */
 
 void
 target_stack::push (target_ops *t)
 {
-  /* If there's already a target at this stratum, remove it.  */
+  t->incref ();
+
   strata stratum = t->stratum ();
 
+  /* If there's already a target at this stratum, remove it.  */
+
   if (m_stack[stratum] != NULL)
     {
       target_ops *prev = m_stack[stratum];
       m_stack[stratum] = NULL;
-      target_close (prev);
+      decref_target (prev);
     }
 
   /* Now add the new one.  */
@@ -580,15 +594,15 @@  target_stack::push (target_ops *t)
 void
 push_target (struct target_ops *t)
 {
-  g_target_stack.push (t);
+  current_inferior ()->push_target (t);
 }
 
-/* See target.h  */
+/* See target.h.  */
 
 void
 push_target (target_ops_up &&t)
 {
-  g_target_stack.push (t.get ());
+  current_inferior ()->push_target (t.get ());
   t.release ();
 }
 
@@ -597,7 +611,7 @@  push_target (target_ops_up &&t)
 int
 unpush_target (struct target_ops *t)
 {
-  return g_target_stack.unpush (t);
+  return current_inferior ()->unpush_target (t);
 }
 
 /* See target.h.  */
@@ -629,10 +643,13 @@  target_stack::unpush (target_ops *t)
   if (m_top == stratum)
     m_top = t->beneath ()->stratum ();
 
-  /* Finally close the target.  Note we do this after unchaining, so
-     any target method calls from within the target_close
-     implementation don't end up in T anymore.  */
-  target_close (t);
+  /* Finally close the target, if there are no inferiors
+     referencing this target still.  Note we do this after unchaining,
+     so any target method calls from within the target_close
+     implementation don't end up in T anymore.  Do leave the target
+     open if we have are other inferiors referencing this target
+     still.  */
+  decref_target (t);
 
   return true;
 }
@@ -674,12 +691,13 @@  pop_all_targets (void)
   pop_all_targets_above (dummy_stratum);
 }
 
-/* Return 1 if T is now pushed in the target stack.  Return 0 otherwise.  */
+/* Return true if T is now pushed in the current inferior's target
+   stack.  Return false otherwise.  */
 
-int
-target_is_pushed (struct target_ops *t)
+bool
+target_is_pushed (target_ops *t)
 {
-  return g_target_stack.is_pushed (t);
+  return current_inferior ()->target_is_pushed (t);
 }
 
 /* Default implementation of to_get_thread_local_address.  */
@@ -1956,14 +1974,14 @@  target_pre_inferior (int from_tty)
 /* Callback for iterate_over_inferiors.  Gets rid of the given
    inferior.  */
 
-static int
-dispose_inferior (struct inferior *inf, void *args)
+static void
+dispose_inferior (inferior *inf)
 {
   /* Not all killed inferiors can, or will ever be, removed from the
      inferior list.  Killed inferiors clearly don't need to be killed
      again, so, we're done.  */
   if (inf->pid == 0)
-    return 0;
+    return;
 
   thread_info *thread = any_thread_of_inferior (inf);
   if (thread != NULL)
@@ -1976,8 +1994,6 @@  dispose_inferior (struct inferior *inf, void *args)
       else
 	target_detach (inf, 0);
     }
-
-  return 0;
 }
 
 /* This is to be called by the open routine before it does
@@ -1988,12 +2004,12 @@  target_preopen (int from_tty)
 {
   dont_repeat ();
 
-  if (have_inferiors ())
+  if (current_inferior ()->pid != 0)
     {
       if (!from_tty
-	  || !have_live_inferiors ()
+	  || !target_has_execution
 	  || query (_("A program is being debugged already.  Kill it? ")))
-	iterate_over_inferiors (dispose_inferior, NULL);
+	dispose_inferior (current_inferior ());
       else
 	error (_("Program not killed."));
     }
@@ -2035,9 +2051,16 @@  target_detach (inferior *inf, int from_tty)
 
   prepare_for_detach ();
 
+  /* Hold a strong reference because detaching may unpush the
+     target.  */
+  auto proc_target_ref = target_ops_ref::new_reference (inf->process_target ());
+
   current_top_target ()->detach (inf, from_tty);
 
-  registers_changed_ptid (save_pid_ptid);
+  process_stratum_target *proc_target
+    = as_process_stratum_target (proc_target_ref.get ());
+
+  registers_changed_ptid (proc_target, save_pid_ptid);
 
   /* We have to ensure we have no frame cache left.  Normally,
      registers_changed_ptid (save_pid_ptid) calls reinit_frame_cache when
@@ -2085,6 +2108,8 @@  target_pid_to_str (ptid_t ptid)
 const char *
 target_thread_name (struct thread_info *info)
 {
+  gdb_assert (info->inf == current_inferior ());
+
   return current_top_target ()->thread_name (info);
 }
 
@@ -2108,16 +2133,18 @@  target_thread_info_to_thread_handle (struct thread_info *tip)
 void
 target_resume (ptid_t ptid, int step, enum gdb_signal signal)
 {
+  process_stratum_target *curr_target = current_inferior ()->process_target ();
+
   target_dcache_invalidate ();
 
   current_top_target ()->resume (ptid, step, signal);
 
-  registers_changed_ptid (ptid);
+  registers_changed_ptid (curr_target, ptid);
   /* We only set the internal executing state here.  The user/frontend
      running state is set at a higher level.  This also clears the
      thread's stop_pc as side effect.  */
-  set_executing (ptid, 1);
-  clear_inline_frame_state (ptid);
+  set_executing (curr_target, ptid, 1);
+  clear_inline_frame_state (curr_target, ptid);
 }
 
 /* If true, target_commit_resume is a nop.  */
@@ -2542,7 +2569,6 @@  target_get_osdata (const char *type)
   return target_read_stralloc (t, TARGET_OBJECT_OSDATA, type);
 }
 
-
 /* Determine the current address space of thread PTID.  */
 
 struct address_space *
@@ -2561,7 +2587,7 @@  target_thread_address_space (ptid_t ptid)
 target_ops *
 target_ops::beneath () const
 {
-  return g_target_stack.find_beneath (this);
+  return current_inferior ()->find_target_beneath (this);
 }
 
 void
@@ -3157,7 +3183,7 @@  target_stack::find_beneath (const target_ops *t) const
 struct target_ops *
 find_target_at (enum strata stratum)
 {
-  return g_target_stack.at (stratum);
+  return current_inferior ()->target_at (stratum);
 }
 
 
@@ -3253,6 +3279,14 @@  dummy_make_corefile_notes (struct target_ops *self,
 
 static dummy_target the_dummy_target;
 
+/* See target.h.  */
+
+target_ops *
+get_dummy_target ()
+{
+  return &the_dummy_target;
+}
+
 static const target_info dummy_target_info = {
   "None",
   N_("None"),
@@ -3339,7 +3373,33 @@  target_interrupt ()
 void
 target_pass_ctrlc (void)
 {
-  current_top_target ()->pass_ctrlc ();
+  /* Pass the Ctrl-C to the first inferior that has a thread
+     running.  */
+  for (inferior *inf : all_inferiors ())
+    {
+      target_ops *proc_target = inf->process_target ();
+      if (proc_target == NULL)
+	continue;
+
+      for (thread_info *thr : inf->threads ())
+	{
+	  /* A thread can be THREAD_STOPPED and executing, while
+	     running an infcall.  */
+	  if (thr->state == THREAD_RUNNING || thr->executing)
+	    {
+	      /* We can get here quite deep in target layers.  Avoid
+		 switching thread context or anything that would
+		 communicate with the target (e.g., to fetch
+		 registers), or flushing e.g., the frame cache.  We
+		 just switch inferior in order to be able to call
+		 through the target_stack.  */
+	      scoped_restore_current_inferior restore_inferior;
+	      set_current_inferior (inf);
+	      current_top_target ()->pass_ctrlc ();
+	      return;
+	    }
+	}
+    }
 }
 
 /* See target.h.  */
@@ -3987,10 +4047,8 @@  set_write_memory_permission (const char *args, int from_tty,
 }
 
 void
-initialize_targets (void)
+_initialize_target (void)
 {
-  push_target (&the_dummy_target);
-
   the_debug_target = new debug_target ();
 
   add_info ("target", info_target_command, targ_desc);
diff --git a/gdb/target.h b/gdb/target.h
index 9a94227953..9ceea7e06a 100644
--- a/gdb/target.h
+++ b/gdb/target.h
@@ -43,6 +43,7 @@  struct inferior;
 #include "infrun.h" /* For enum exec_direction_kind.  */
 #include "breakpoint.h" /* For enum bptype.  */
 #include "gdbsupport/scoped_restore.h"
+#include "gdbsupport/refcounted-object.h"
 
 /* This include file defines the interface between the main part
    of the debugger, and the part which is target-specific, or
@@ -430,6 +431,7 @@  struct target_info
 };
 
 struct target_ops
+  : public refcounted_object
   {
     /* Return this target's stratum.  */
     virtual strata stratum () const = 0;
@@ -448,10 +450,10 @@  struct target_ops
     virtual const target_info &info () const = 0;
 
     /* Name this target type.  */
-    const char *shortname ()
+    const char *shortname () const
     { return info ().shortname; }
 
-    const char *longname ()
+    const char *longname () const
     { return info ().longname; }
 
     /* Close the target.  This is where the target can handle
@@ -697,6 +699,8 @@  struct target_ops
       TARGET_DEFAULT_RETURN (false);
     virtual void async (int)
       TARGET_DEFAULT_NORETURN (tcomplain ());
+    virtual int async_wait_fd ()
+      TARGET_DEFAULT_NORETURN (noprocess ());
     virtual void thread_events (int)
       TARGET_DEFAULT_IGNORE ();
     /* This method must be implemented in some situations.  See the
@@ -1261,6 +1265,9 @@  struct target_ops_deleter
 /* A unique pointer for target_ops.  */
 typedef std::unique_ptr<target_ops, target_ops_deleter> target_ops_up;
 
+/* A gdb::ref_ptr pointer to a target_ops.  */
+typedef gdb::ref_ptr<target_ops, refcounted_object_ref_policy> target_ops_ref;
+
 /* Native target backends call this once at initialization time to
    inform the core about which is the target that can respond to "run"
    or "attach".  Note: native targets are always singletons.  */
@@ -1315,6 +1322,9 @@  private:
 
 extern target_ops *current_top_target ();
 
+/* Return the dummy target.  */
+extern target_ops *get_dummy_target ();
+
 /* Define easy words for doing these operations on our current target.  */
 
 #define	target_shortname	(current_top_target ()->shortname ())
@@ -2363,7 +2373,7 @@  extern void pop_all_targets_at_and_above (enum strata stratum);
    strictly above ABOVE_STRATUM.  */
 extern void pop_all_targets_above (enum strata above_stratum);
 
-extern int target_is_pushed (struct target_ops *t);
+extern bool target_is_pushed (target_ops *t);
 
 extern CORE_ADDR target_translate_tls_address (struct objfile *objfile,
 					       CORE_ADDR offset);
diff --git a/gdb/thread-iter.c b/gdb/thread-iter.c
index 9a41a46aa6..a41f894325 100644
--- a/gdb/thread-iter.c
+++ b/gdb/thread-iter.c
@@ -58,16 +58,22 @@  all_threads_iterator::advance ()
 bool
 all_matching_threads_iterator::m_inf_matches ()
 {
-  return (m_filter_ptid == minus_one_ptid
-	  || m_filter_ptid.pid () == m_inf->pid);
+  return ((m_filter_target == nullptr
+	   || m_filter_target == m_inf->process_target ())
+	  && (m_filter_ptid == minus_one_ptid
+	      || m_filter_ptid.pid () == m_inf->pid));
 }
 
 /* See thread-iter.h.  */
 
 all_matching_threads_iterator::all_matching_threads_iterator
-  (ptid_t filter_ptid)
-  : m_filter_ptid (filter_ptid)
+  (process_stratum_target *filter_target, ptid_t filter_ptid)
+    : m_filter_target (filter_target),
+      m_filter_ptid (filter_ptid)
 {
+  gdb_assert ((filter_target == nullptr && filter_ptid == minus_one_ptid)
+	      || filter_target->stratum () == process_stratum);
+
   m_thr = nullptr;
   for (m_inf = inferior_list; m_inf != NULL; m_inf = m_inf->next)
     if (m_inf_matches ())
diff --git a/gdb/thread-iter.h b/gdb/thread-iter.h
index 72ee9ddcb8..8586653df2 100644
--- a/gdb/thread-iter.h
+++ b/gdb/thread-iter.h
@@ -92,12 +92,14 @@  public:
 
   /* Creates an iterator that iterates over all threads that match
      FILTER_PTID.  */
-  explicit all_matching_threads_iterator (ptid_t filter_ptid);
+  explicit all_matching_threads_iterator (process_stratum_target *filter_target,
+					  ptid_t filter_ptid);
 
   /* Create a one-past-end iterator.  */
   all_matching_threads_iterator ()
     : m_inf (nullptr),
       m_thr (nullptr),
+      m_filter_target (nullptr),
       m_filter_ptid (minus_one_ptid)
   {}
 
@@ -131,6 +133,7 @@  private:
   thread_info *m_thr;
 
   /* The filter.  */
+  process_stratum_target *m_filter_target;
   ptid_t m_filter_ptid;
 };
 
@@ -211,20 +214,22 @@  struct all_threads_safe_range
 struct all_matching_threads_range
 {
 public:
-  explicit all_matching_threads_range (ptid_t filter_ptid)
-    : m_filter_ptid (filter_ptid)
+  explicit all_matching_threads_range (process_stratum_target *filter_target,
+				       ptid_t filter_ptid)
+    : m_filter_target (filter_target), m_filter_ptid (filter_ptid)
   {}
   all_matching_threads_range ()
-    : m_filter_ptid (minus_one_ptid)
+    : m_filter_target (nullptr), m_filter_ptid (minus_one_ptid)
   {}
 
   all_matching_threads_iterator begin () const
-  { return all_matching_threads_iterator (m_filter_ptid); }
+  { return all_matching_threads_iterator (m_filter_target, m_filter_ptid); }
   all_matching_threads_iterator end () const
   { return all_matching_threads_iterator (); }
 
 private:
   /* The filter.  */
+  process_stratum_target *m_filter_target;
   ptid_t m_filter_ptid;
 };
 
@@ -236,20 +241,22 @@  private:
 class all_non_exited_threads_range
 {
 public:
-  explicit all_non_exited_threads_range (ptid_t filter_ptid)
-    : m_filter_ptid (filter_ptid)
+  explicit all_non_exited_threads_range (process_stratum_target *filter_target,
+					 ptid_t filter_ptid)
+    : m_filter_target (filter_target), m_filter_ptid (filter_ptid)
   {}
 
   all_non_exited_threads_range ()
-    : m_filter_ptid (minus_one_ptid)
+    : m_filter_target (nullptr), m_filter_ptid (minus_one_ptid)
   {}
 
   all_non_exited_threads_iterator begin () const
-  { return all_non_exited_threads_iterator (m_filter_ptid); }
+  { return all_non_exited_threads_iterator (m_filter_target, m_filter_ptid); }
   all_non_exited_threads_iterator end () const
   { return all_non_exited_threads_iterator (); }
 
 private:
+  process_stratum_target *m_filter_target;
   ptid_t m_filter_ptid;
 };
 
diff --git a/gdb/thread.c b/gdb/thread.c
index 9a5867306b..9dafca57bf 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -55,13 +55,6 @@ 
 
 static int highest_thread_num;
 
-/* True if any thread is, or may be executing.  We need to track this
-   separately because until we fully sync the thread list, we won't
-   know whether the target is fully stopped, even if we see stop
-   events for all known threads, because any of those threads may have
-   spawned new threads we haven't heard of yet.  */
-static int threads_executing;
-
 /* RAII type used to increase / decrease the refcount of each thread
    in a given list of threads.  */
 
@@ -89,7 +82,7 @@  private:
 struct thread_info*
 inferior_thread (void)
 {
-  struct thread_info *tp = find_thread_ptid (inferior_ptid);
+  struct thread_info *tp = find_thread_ptid (current_inferior (), inferior_ptid);
   gdb_assert (tp);
   return tp;
 }
@@ -195,7 +188,7 @@  clear_thread_inferior_resources (struct thread_info *tp)
 
   thread_cancel_execution_command (tp);
 
-  clear_inline_frame_state (tp->ptid);
+  clear_inline_frame_state (tp);
 }
 
 /* Set the TP's state as exited.  */
@@ -260,12 +253,11 @@  new_thread (struct inferior *inf, ptid_t ptid)
 }
 
 struct thread_info *
-add_thread_silent (ptid_t ptid)
+add_thread_silent (process_stratum_target *targ, ptid_t ptid)
 {
-  struct inferior *inf = find_inferior_ptid (ptid);
-  gdb_assert (inf != NULL);
+  inferior *inf;
 
-  thread_info *tp = find_thread_ptid (inf, ptid);
+  thread_info *tp = find_thread_ptid (targ, ptid);
   if (tp)
     /* Found an old thread with the same id.  It has to be dead,
        otherwise we wouldn't be adding a new thread with the same id.
@@ -281,7 +273,7 @@  add_thread_silent (ptid_t ptid)
 
       if (inferior_ptid == ptid)
 	{
-	  thread_info *new_thr = new_thread (inf, null_ptid);
+	  thread_info *new_thr = new_thread (tp->inf, null_ptid);
 
 	  /* Make switch_to_thread not read from the thread.  */
 	  new_thr->state = THREAD_EXITED;
@@ -300,10 +292,14 @@  add_thread_silent (ptid_t ptid)
 	  /* All done.  */
 	  return new_thr;
 	}
-      else
-	/* Just go ahead and delete it.  */
-	delete_thread (tp);
+
+      inf = tp->inf;
+
+      /* Just go ahead and delete it.  */
+      delete_thread (tp);
     }
+  else
+    inf = find_inferior_ptid (targ, ptid);
 
   tp = new_thread (inf, ptid);
   gdb::observers::new_thread.notify (tp);
@@ -312,9 +308,10 @@  add_thread_silent (ptid_t ptid)
 }
 
 struct thread_info *
-add_thread_with_info (ptid_t ptid, private_thread_info *priv)
+add_thread_with_info (process_stratum_target *targ, ptid_t ptid,
+		      private_thread_info *priv)
 {
-  struct thread_info *result = add_thread_silent (ptid);
+  thread_info *result = add_thread_silent (targ, ptid);
 
   result->priv.reset (priv);
 
@@ -326,9 +323,9 @@  add_thread_with_info (ptid_t ptid, private_thread_info *priv)
 }
 
 struct thread_info *
-add_thread (ptid_t ptid)
+add_thread (process_stratum_target *targ, ptid_t ptid)
 {
-  return add_thread_with_info (ptid, NULL);
+  return add_thread_with_info (targ, ptid, NULL);
 }
 
 private_thread_info::~private_thread_info () = default;
@@ -352,6 +349,14 @@  thread_info::~thread_info ()
   xfree (this->name);
 }
 
+/* Returns true if THR is the current thread.  */
+
+static bool
+is_current_thread (const thread_info *thr)
+{
+  return thr->inf == current_inferior () && thr->ptid == inferior_ptid;
+}
+
 /* See gdbthread.h.  */
 
 bool
@@ -359,7 +364,7 @@  thread_info::deletable () const
 {
   /* If this is the current thread, or there's code out there that
      relies on it existing (refcount > 0) we can't delete yet.  */
-  return refcount () == 0 && ptid != inferior_ptid;
+  return refcount () == 0 && !is_current_thread (this);
 }
 
 /* Add TP to the end of the step-over chain LIST_P.  */
@@ -514,12 +519,12 @@  find_thread_id (struct inferior *inf, int thr_num)
   return NULL;
 }
 
-/* Find a thread_info by matching PTID.  */
+/* See gdbthread.h.  */
 
 struct thread_info *
-find_thread_ptid (ptid_t ptid)
+find_thread_ptid (process_stratum_target *targ, ptid_t ptid)
 {
-  inferior *inf = find_inferior_ptid (ptid);
+  inferior *inf = find_inferior_ptid (targ, ptid);
   if (inf == NULL)
     return NULL;
   return find_thread_ptid (inf, ptid);
@@ -584,9 +589,9 @@  any_thread_p ()
 }
 
 int
-thread_count (void)
+thread_count (process_stratum_target *proc_target)
 {
-  auto rng = all_threads ();
+  auto rng = all_threads (proc_target);
   return std::distance (rng.begin (), rng.end ());
 }
 
@@ -609,10 +614,10 @@  valid_global_thread_id (int global_id)
   return 0;
 }
 
-int
-in_thread_list (ptid_t ptid)
+bool
+in_thread_list (process_stratum_target *targ, ptid_t ptid)
 {
-  return find_thread_ptid (ptid) != nullptr;
+  return find_thread_ptid (targ, ptid) != nullptr;
 }
 
 /* Finds the first thread of the inferior.  */
@@ -788,7 +793,8 @@  get_last_thread_stack_temporary (thread_info *tp)
 }
 
 void
-thread_change_ptid (ptid_t old_ptid, ptid_t new_ptid)
+thread_change_ptid (process_stratum_target *targ,
+		    ptid_t old_ptid, ptid_t new_ptid)
 {
   struct inferior *inf;
   struct thread_info *tp;
@@ -796,7 +802,7 @@  thread_change_ptid (ptid_t old_ptid, ptid_t new_ptid)
   /* It can happen that what we knew as the target inferior id
      changes.  E.g, target remote may only discover the remote process
      pid after adding the inferior to GDB's list.  */
-  inf = find_inferior_ptid (old_ptid);
+  inf = find_inferior_ptid (targ, old_ptid);
   inf->pid = new_ptid.pid ();
 
   tp = find_thread_ptid (inf, old_ptid);
@@ -808,9 +814,9 @@  thread_change_ptid (ptid_t old_ptid, ptid_t new_ptid)
 /* See gdbthread.h.  */
 
 void
-set_resumed (ptid_t ptid, int resumed)
+set_resumed (process_stratum_target *targ, ptid_t ptid, bool resumed)
 {
-  for (thread_info *tp : all_non_exited_threads (ptid))
+  for (thread_info *tp : all_non_exited_threads (targ, ptid))
     tp->resumed = resumed;
 }
 
@@ -848,7 +854,7 @@  thread_info::set_running (bool running)
 }
 
 void
-set_running (ptid_t ptid, int running)
+set_running (process_stratum_target *targ, ptid_t ptid, bool running)
 {
   /* We try not to notify the observer if no thread has actually
      changed the running state -- merely to reduce the number of
@@ -856,7 +862,7 @@  set_running (ptid_t ptid, int running)
      multiple *running notifications just fine.  */
   bool any_started = false;
 
-  for (thread_info *tp : all_non_exited_threads (ptid))
+  for (thread_info *tp : all_non_exited_threads (targ, ptid))
     if (set_running_thread (tp, running))
       any_started = true;
 
@@ -878,32 +884,32 @@  set_executing_thread (thread_info *thr, bool executing)
 }
 
 void
-set_executing (ptid_t ptid, int executing)
+set_executing (process_stratum_target *targ, ptid_t ptid, bool executing)
 {
-  for (thread_info *tp : all_non_exited_threads (ptid))
+  for (thread_info *tp : all_non_exited_threads (targ, ptid))
     set_executing_thread (tp, executing);
 
   /* It only takes one running thread to spawn more threads.  */
   if (executing)
-    threads_executing = 1;
+    targ->threads_executing = true;
   /* Only clear the flag if the caller is telling us everything is
      stopped.  */
   else if (minus_one_ptid == ptid)
-    threads_executing = 0;
+    targ->threads_executing = false;
 }
 
 /* See gdbthread.h.  */
 
-int
-threads_are_executing (void)
+bool
+threads_are_executing (process_stratum_target *target)
 {
-  return threads_executing;
+  return target->threads_executing;
 }
 
 void
-set_stop_requested (ptid_t ptid, int stop)
+set_stop_requested (process_stratum_target *targ, ptid_t ptid, bool stop)
 {
-  for (thread_info *tp : all_non_exited_threads (ptid))
+  for (thread_info *tp : all_non_exited_threads (targ, ptid))
     tp->stop_requested = stop;
 
   /* Call the stop requested observer so other components of GDB can
@@ -913,11 +919,11 @@  set_stop_requested (ptid_t ptid, int stop)
 }
 
 void
-finish_thread_state (ptid_t ptid)
+finish_thread_state (process_stratum_target *targ, ptid_t ptid)
 {
   bool any_started = false;
 
-  for (thread_info *tp : all_non_exited_threads (ptid))
+  for (thread_info *tp : all_non_exited_threads (targ, ptid))
     if (set_running_thread (tp, tp->executing))
       any_started = true;
 
@@ -1333,7 +1339,7 @@  switch_to_thread (thread_info *thr)
 {
   gdb_assert (thr != NULL);
 
-  if (inferior_ptid == thr->ptid)
+  if (is_current_thread (thr))
     return;
 
   switch_to_thread_no_regs (thr);
@@ -1344,9 +1350,9 @@  switch_to_thread (thread_info *thr)
 /* See gdbsupport/common-gdbthread.h.  */
 
 void
-switch_to_thread (ptid_t ptid)
+switch_to_thread (process_stratum_target *proc_target, ptid_t ptid)
 {
-  thread_info *thr = find_thread_ptid (ptid);
+  thread_info *thr = find_thread_ptid (proc_target, ptid);
   switch_to_thread (thr);
 }
 
@@ -2069,18 +2075,39 @@  print_selected_thread_frame (struct ui_out *uiout,
 }
 
 /* Update the 'threads_executing' global based on the threads we know
-   about right now.  */
+   about right now.  This is used by infrun to tell whether we should
+   pull events out of the current target.  */
 
 static void
 update_threads_executing (void)
 {
-  threads_executing = 0;
-  for (thread_info *tp : all_non_exited_threads ())
+  process_stratum_target *targ = current_inferior ()->process_target ();
+
+  if (targ == NULL)
+    return;
+
+  targ->threads_executing = false;
+
+  for (inferior *inf : all_non_exited_inferiors (targ))
     {
-      if (tp->executing)
+      if (!inf->has_execution ())
+	continue;
+
+      /* If the process has no threads, then it must be we have a
+	 process-exit event pending.  */
+      if (inf->thread_list == NULL)
+	{
+	  targ->threads_executing = true;
+	  return;
+	}
+
+      for (thread_info *tp : inf->non_exited_threads ())
 	{
-	  threads_executing = 1;
-	  break;
+	  if (tp->executing)
+	    {
+	      targ->threads_executing = true;
+	      return;
+	    }
 	}
     }
 }
diff --git a/gdb/top.c b/gdb/top.c
index 9d4ce1fa3b..6af8c1629e 100644
--- a/gdb/top.c
+++ b/gdb/top.c
@@ -1681,13 +1681,17 @@  quit_force (int *exit_arg, int from_tty)
 
   /* Give all pushed targets a chance to do minimal cleanup, and pop
      them all out.  */
-  try
+  for (inferior *inf : all_inferiors ())
     {
-      pop_all_targets ();
-    }
-  catch (const gdb_exception &ex)
-    {
-      exception_print (gdb_stderr, ex);
+      switch_to_inferior_no_thread (inf);
+      try
+	{
+	  pop_all_targets ();
+	}
+      catch (const gdb_exception &ex)
+	{
+	  exception_print (gdb_stderr, ex);
+	}
     }
 
   /* Save the history information if it is appropriate to do so.  */
@@ -2233,7 +2237,6 @@  gdb_init (char *argv0)
 #endif
 
   init_cmd_lists ();	    /* This needs to be done first.  */
-  initialize_targets ();    /* Setup target_terminal macros for utils.c.  */
   initialize_utils ();	    /* Make errors and warnings possible.  */
 
   init_page_info ();
diff --git a/gdb/tracefile-tfile.c b/gdb/tracefile-tfile.c
index c5063e66b0..f6ff71104f 100644
--- a/gdb/tracefile-tfile.c
+++ b/gdb/tracefile-tfile.c
@@ -557,7 +557,7 @@  tfile_target_open (const char *arg, int from_tty)
 
   inferior_appeared (current_inferior (), TFILE_PID);
   inferior_ptid = ptid_t (TFILE_PID);
-  add_thread_silent (inferior_ptid);
+  add_thread_silent (&tfile_ops, inferior_ptid);
 
   if (ts->traceframe_count <= 0)
     warning (_("No traceframes present in this file."));