[RFC,0/3,gdb/symtab] Fix data-races in gdb.base/index-cache.exp

  When building gdb with -fsanitize=thread, we run into a data race in
gdb.base/index-cache.exp.

Fixing this leads us to another, and another, so there are three patches, each
fixing a data race.

The first two patches implement the approach mentioned in PR30392 comment 2:
...
The reader probably should capture the necessarily globals
on the main thread and stash them until the index has been
written.
...

The 3rd patch cannot be fixed with this approach, so it uses the std::atomic
approach:
...
-  unsigned int queued : 1;
+  std::atomic<bool> queued;
...

The 3rd patch is the reason this is an RFC.

I spent some time convincing myself that this is not a benign data race.  I
started with reading [1], and got convinced by "2.5 Disjoint bit manipulation"
in [2].  Also [3] looked interesting, but haven't read it in full.

As for fixing it, the std::atomic approach works.  I checked that the struct
size is not increased.  It gives me an almost clean test-suite run (still
running into PR30680), though I'm running into a lot of timeouts, so things
still may be hidden there.

Alternatively, if I limit the fix to:
...
-  unsigned int queued : 1;
+  bool queued;
...
I run into 19 more reported data races in the test-case.  None of them look
as evidence that this fix on queued is insufficient.

Looking at the first one (see appendix for more detail), there's a race
between:
- the main thread calling find_main_name, because of calling main_language in
  set_initial_language, and
- the worker thread calling find_main_name, because of calling main_name in
  write_cooked_index.
At least 14 other of the 19 reported data races look like instances of the
first one.

This looks like a real problem to me, but with the proposed fix it no longer
triggers, although I fail to see how it fixes it.

Which leaves me with the questions:
- is the std::atomic approach too blunt, and are we consequently hiding
  problems, and
- should we address the find_main_name race in some form or another.

Tested on x86_64-linux, with and without -fsanitize=thread.

PR symtab/30392
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30392

References:
[1] https://hacks.mozilla.org/2021/04/eliminating-data-races-in-firefox-a-technical-report/
[2] https://www.usenix.org/legacy/event/hotpar11/tech/final_files/Boehm.pdf
[3] https://bartoszmilewski.com/2020/08/11/benign-data-races-considered-harmful/

Appendix:
...
(gdb) file build/gdb/testsuite/outputs/gdb.base/index-cache/index-cache
Reading symbols from build/gdb/testsuite/outputs/gdb.base/index-cache/index-cache...
==================
WARNING: ThreadSanitizer: data race (pid=1569)
  Read of size 1 at 0x7b4400097d08 by thread T12:
    #0 bfd_get_section_limit_octets bfd.h:2376 (gdb+0x149b613)
    #1 bfd_get_section_contents bfd/section.c:1571 (gdb+0x149cbe4)
    #2 gdb_bfd_scan_elf_dyntag(int, bfd*, unsigned long*, unsigned long*) gdb/solib.c:1601 (gdb+0xed8fc8)
    #3 elf_locate_base gdb/solib-svr4.c:705 (gdb+0xec29aa)
    #4 svr4_iterate_over_objfiles_in_search_order gdb/solib-svr4.c:3430 (gdb+0xeca65b)
    #5 gdbarch_iterate_over_objfiles_in_search_order(gdbarch*, gdb::function_view<bool (objfile*)>, objfile*) gdb/gdbarch.c:5041 (gdb+0x537cad)
    #6 find_main_name gdb/symtab.c:6270 (gdb+0xf744a3)
    #7 main_name() gdb/symtab.c:6299 (gdb+0xf74531)
    #8 write_cooked_index gdb/dwarf2/index-write.c:1131 (gdb+0x830af4)
    #9 write_gdbindex gdb/dwarf2/index-write.c:1256 (gdb+0x83134b)
    #10 write_dwarf_index(dwarf2_per_bfd*, char const*, char const*, char const*, dw_index_kind) gdb/dwarf2/index-write.c:1484 (gdb+0x83232f)
    #11 index_cache::store(dwarf2_per_bfd*, index_cache_store_context*) gdb/dwarf2/index-cache.c:177 (gdb+0x82d62b)
    #12 cooked_index::maybe_write_index(dwarf2_per_bfd*) gdb/dwarf2/cooked-index.c:640 (gdb+0x7f1bf7)
    #13 operator() gdb/dwarf2/cooked-index.c:470 (gdb+0x7f0f40)
    #14 _M_invoke /usr/include/c++/7/bits/std_function.h:316 (gdb+0x7f2909)
    #15 std::function<void ()>::operator()() const /usr/include/c++/7/bits/std_function.h:706 (gdb+0x700952)
    #16 void std::__invoke_impl<void, std::function<void ()>&>(std::__invoke_other, std::function<void ()>&) /usr/include/c++/7/bits/invoke.h:60 (gdb+0x7381a0)
    #17 std::__invoke_result<std::function<void ()>&>::type std::__invoke<std::function<void ()>&>(std::function<void ()>&) /usr/include/c++/7/bits/invoke.h:95 (gdb+0x737e91)
    #18 std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run()::{lambda()#1}::operator()() const /usr/include/c++/7/future:1421 (gdb+0x737b59)
    #19 std::__future_base::_Task_setter<std::unique_ptr<std::__future_base::_Result<void>, std::__future_base::_Result_base::_Deleter>, std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run()::{lambda()#1}, void>::operator()() const /usr/include/c++/7/future:1362 (gdb+0x738660)
    #20 std::_Function_handler<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> (), std::__future_base::_Task_setter<std::unique_ptr<std::__future_base::_Result<void>, std::__future_base::_Result_base::_Deleter>, std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run()::{lambda()#1}, void> >::_M_invoke(std::_Any_data const&) /usr/include/c++/7/bits/std_function.h:302 (gdb+0x73825c)
    #21 std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>::operator()() const /usr/include/c++/7/bits/std_function.h:706 (gdb+0x733623)
    #22 std::__future_base::_State_baseV2::_M_do_set(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*) /usr/include/c++/7/future:561 (gdb+0x732bdf)
    #23 void std::__invoke_impl<void, void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::__invoke_memfun_deref, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) /usr/include/c++/7/bits/invoke.h:73 (gdb+0x734c4f)
    #24 std::__invoke_result<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>::type std::__invoke<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) /usr/include/c++/7/bits/invoke.h:95 (gdb+0x733bc5)
    #25 std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&)::{lambda()#1}::operator()() const /usr/include/c++/7/mutex:672 (gdb+0x73300d)
    #26 std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&)::{lambda()#2}::operator()() const /usr/include/c++/7/mutex:677 (gdb+0x7330b2)
    #27 std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&)::{lambda()#2}::_FUN() /usr/include/c++/7/mutex:677 (gdb+0x7330f2)
    #28 pthread_once <null> (libtsan.so.0+0x4457c)
    #29 __gthread_once /usr/include/c++/7/x86_64-suse-linux/bits/gthr-default.h:699 (gdb+0x72f5dd)
    #30 void std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) /usr/include/c++/7/mutex:684 (gdb+0x733224)
    #31 std::__future_base::_State_baseV2::_M_set_result(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>, bool) /usr/include/c++/7/future:401 (gdb+0x732852)
    #32 std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run() /usr/include/c++/7/future:1423 (gdb+0x737bef)
    #33 std::packaged_task<void ()>::operator()() /usr/include/c++/7/future:1556 (gdb+0x1daca1c)
    #34 gdb::thread_pool::thread_function() gdbsupport/thread-pool.cc:242 (gdb+0x1dac33e)
    #35 void std::__invoke_impl<void, void (gdb::thread_pool::*)(), gdb::thread_pool*>(std::__invoke_memfun_deref, void (gdb::thread_pool::*&&)(), gdb::thread_pool*&&) /usr/include/c++/7/bits/invoke.h:73 (gdb+0x1dad3ed)
    #36 std::__invoke_result<void (gdb::thread_pool::*)(), gdb::thread_pool*>::type std::__invoke<void (gdb::thread_pool::*)(), gdb::thread_pool*>(void (gdb::thread_pool::*&&)(), gdb::thread_pool*&&) /usr/include/c++/7/bits/invoke.h:95 (gdb+0x1dac81e)
    #37 decltype (__invoke((_S_declval<0ul>)(), (_S_declval<1ul>)())) std::thread::_Invoker<std::tuple<void (gdb::thread_pool::*)(), gdb::thread_pool*> >::_M_invoke<0ul, 1ul>(std::_Index_tuple<0ul, 1ul>) /usr/include/c++/7/thread:234 (gdb+0x1dafb50)
    #38 std::thread::_Invoker<std::tuple<void (gdb::thread_pool::*)(), gdb::thread_pool*> >::operator()() /usr/include/c++/7/thread:243 (gdb+0x1dafadb)
    #39 std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (gdb::thread_pool::*)(), gdb::thread_pool*> > >::_M_run() /usr/include/c++/7/thread:186 (gdb+0x1dafa90)
    #40 <null> <null> (libstdc++.so.6+0xdcac2)

  Previous write of size 4 at 0x7b4400097d08 by main thread:
    #0 bfd_open_file bfd/cache.c:584 (gdb+0x148bc90)
    #1 bfd_cache_lookup_worker bfd/cache.c:261 (gdb+0x148b228)
    #2 cache_bseek bfd/cache.c:289 (gdb+0x148b422)
    #3 bfd_seek bfd/bfdio.c:459 (gdb+0x1489d2f)
    #4 _bfd_generic_get_section_contents bfd/libbfd.c:1069 (gdb+0x14978a2)
    #5 bfd_get_section_contents bfd/section.c:1606 (gdb+0x149cd7a)
    #6 gdb_bfd_scan_elf_dyntag(int, bfd*, unsigned long*, unsigned long*) gdb/solib.c:1601 (gdb+0xed8fc8)
    #7 elf_locate_base gdb/solib-svr4.c:705 (gdb+0xec29aa)
    #8 svr4_iterate_over_objfiles_in_search_order gdb/solib-svr4.c:3430 (gdb+0xeca65b)
    #9 gdbarch_iterate_over_objfiles_in_search_order(gdbarch*, gdb::function_view<bool (objfile*)>, objfile*) gdb/gdbarch.c:5041 (gdb+0x537cad)
    #10 find_main_name gdb/symtab.c:6270 (gdb+0xf744a3)
    #11 main_language() gdb/symtab.c:6313 (gdb+0xf74597)
    #12 set_initial_language() gdb/symfile.c:1700 (gdb+0xf4295a)
    #13 symbol_file_add_main_1 gdb/symfile.c:1212 (gdb+0xf40f28)
    #14 symbol_file_command(char const*, int) gdb/symfile.c:1681 (gdb+0xf428cf)
    #15 file_command gdb/exec.c:554 (gdb+0x94f849)
    #16 do_simple_func gdb/cli/cli-decode.c:95 (gdb+0x6d9528)
    #17 cmd_func(cmd_list_element*, char const*, int) gdb/cli/cli-decode.c:2735 (gdb+0x6e0f69)
    #18 execute_command(char const*, int) gdb/top.c:575 (gdb+0xff313a)
    #19 command_handler(char const*) gdb/event-top.c:552 (gdb+0x94aedc)
    #20 command_line_handler(std::unique_ptr<char, gdb::xfree_deleter<char> >&&) gdb/event-top.c:788 (gdb+0x94b599)
    #21 tui_command_line_handler gdb/tui/tui-interp.c:104 (gdb+0x103489a)
    #22 gdb_rl_callback_handler gdb/event-top.c:259 (gdb+0x94a481)
    #23 rl_callback_read_char readline/readline/callback.c:290 (gdb+0x11bdf5b)
    #24 gdb_rl_callback_read_char_wrapper_noexcept gdb/event-top.c:195 (gdb+0x94a280)
    #25 gdb_rl_callback_read_char_wrapper gdb/event-top.c:234 (gdb+0x94a341)
    #26 stdin_event_handler gdb/ui.c:155 (gdb+0x1074b3e)
    #27 handle_file_event gdbsupport/event-loop.cc:573 (gdb+0x1d9536e)
    #28 gdb_wait_for_event gdbsupport/event-loop.cc:694 (gdb+0x1d95aa6)
    #29 gdb_do_one_event(int) gdbsupport/event-loop.cc:264 (gdb+0x1d93e92)
    #30 start_event_loop gdb/main.c:412 (gdb+0xb5a472)
    #31 captured_command_loop gdb/main.c:476 (gdb+0xb5a661)
    #32 captured_main gdb/main.c:1320 (gdb+0xb5c7e1)
    #33 gdb_main(captured_main_args*) gdb/main.c:1339 (gdb+0xb5c890)
    #34 main gdb/gdb.c:32 (gdb+0x416776)
  ...
SUMMARY: ThreadSanitizer: data race bfd.h:2376 in bfd_get_section_limit_octets
...

Tom de Vries (3):
  [gdb/symtab] Fix data race in index_cache::enable
  [gdb/symtab] Fix data race in bfd_open_file
  [gdb/symtab] Fix race on per_cu->queued

 gdb/dwarf2/cooked-index.c |  8 ++++--
 gdb/dwarf2/cooked-index.h |  3 ++
 gdb/dwarf2/index-cache.c  | 58 +++++++++++++++++++++++++++++----------
 gdb/dwarf2/index-cache.h  | 20 +++++++++++++-
 gdb/dwarf2/read.h         |  2 +-
 5 files changed, 72 insertions(+), 19 deletions(-)


base-commit: 29c108c9610640439daa5244a573348b7c47d994
  

On 7/28/23 10:56, Tom de Vries via Gdb-patches wrote:
> When building gdb with -fsanitize=thread, we run into a data race in
> gdb.base/index-cache.exp.
> 

I've submitted a v2 here ( 
https://sourceware.org/pipermail/gdb-patches/2023-August/201270.html ).

> Fixing this leads us to another, and another, so there are three patches, each
> fixing a data race.
> 
> The first two patches implement the approach mentioned in PR30392 comment 2:
> ...
> The reader probably should capture the necessarily globals
> on the main thread and stash them until the index has been
> written.
> ...
> 
> The 3rd patch cannot be fixed with this approach, so it uses the std::atomic
> approach:
> ...
> -  unsigned int queued : 1;
> +  std::atomic<bool> queued;
> ...
> 
> The 3rd patch is the reason this is an RFC.
> 

In v2 I'm no longer using std::atomic<bool>, instead I'm using 
packed<bool, 1>.

> I spent some time convincing myself that this is not a benign data race.  I
> started with reading [1], and got convinced by "2.5 Disjoint bit manipulation"
> in [2].  Also [3] looked interesting, but haven't read it in full.
> 
> As for fixing it, the std::atomic approach works.  I checked that the struct
> size is not increased.  It gives me an almost clean test-suite run (still
> running into PR30680), though I'm running into a lot of timeouts, so things
> still may be hidden there.
> 

I now also ran on a fast server with lots of memory, and the timeouts do 
not happen there.

> Alternatively, if I limit the fix to:
> ...
> -  unsigned int queued : 1;
> +  bool queued;
> ...
> I run into 19 more reported data races in the test-case.  None of them look
> as evidence that this fix on queued is insufficient.
> 
> Looking at the first one (see appendix for more detail), there's a race
> between:
> - the main thread calling find_main_name, because of calling main_language in
>    set_initial_language, and
> - the worker thread calling find_main_name, because of calling main_name in
>    write_cooked_index.
> At least 14 other of the 19 reported data races look like instances of the
> first one.
> 
> This looks like a real problem to me, but with the proposed fix it no longer
> triggers, although I fail to see how it fixes it.
> 

I managed to trigger data races by running with taskset -c 0.  I ended 
up fixing data races until the combination of make-check-all.sh and 
taskset -c 0 no longer triggered any.  Indeed one of the patches in v2 
does address the find_main_name issue.

> Which leaves me with the questions:
> - is the std::atomic approach too blunt, and are we consequently hiding
>    problems, and
> - should we address the find_main_name race in some form or another.
> 
I no longer have these questions, so the v2 no longer is an RFC.

Thanks,
- Tom

> Tested on x86_64-linux, with and without -fsanitize=thread.
> 
> PR symtab/30392
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30392
> 
> References:
> [1] https://hacks.mozilla.org/2021/04/eliminating-data-races-in-firefox-a-technical-report/
> [2] https://www.usenix.org/legacy/event/hotpar11/tech/final_files/Boehm.pdf
> [3] https://bartoszmilewski.com/2020/08/11/benign-data-races-considered-harmful/
> 
> Appendix:
> ...
> (gdb) file build/gdb/testsuite/outputs/gdb.base/index-cache/index-cache
> Reading symbols from build/gdb/testsuite/outputs/gdb.base/index-cache/index-cache...
> ==================
> WARNING: ThreadSanitizer: data race (pid=1569)
>    Read of size 1 at 0x7b4400097d08 by thread T12:
>      #0 bfd_get_section_limit_octets bfd.h:2376 (gdb+0x149b613)
>      #1 bfd_get_section_contents bfd/section.c:1571 (gdb+0x149cbe4)
>      #2 gdb_bfd_scan_elf_dyntag(int, bfd*, unsigned long*, unsigned long*) gdb/solib.c:1601 (gdb+0xed8fc8)
>      #3 elf_locate_base gdb/solib-svr4.c:705 (gdb+0xec29aa)
>      #4 svr4_iterate_over_objfiles_in_search_order gdb/solib-svr4.c:3430 (gdb+0xeca65b)
>      #5 gdbarch_iterate_over_objfiles_in_search_order(gdbarch*, gdb::function_view<bool (objfile*)>, objfile*) gdb/gdbarch.c:5041 (gdb+0x537cad)
>      #6 find_main_name gdb/symtab.c:6270 (gdb+0xf744a3)
>      #7 main_name() gdb/symtab.c:6299 (gdb+0xf74531)
>      #8 write_cooked_index gdb/dwarf2/index-write.c:1131 (gdb+0x830af4)
>      #9 write_gdbindex gdb/dwarf2/index-write.c:1256 (gdb+0x83134b)
>      #10 write_dwarf_index(dwarf2_per_bfd*, char const*, char const*, char const*, dw_index_kind) gdb/dwarf2/index-write.c:1484 (gdb+0x83232f)
>      #11 index_cache::store(dwarf2_per_bfd*, index_cache_store_context*) gdb/dwarf2/index-cache.c:177 (gdb+0x82d62b)
>      #12 cooked_index::maybe_write_index(dwarf2_per_bfd*) gdb/dwarf2/cooked-index.c:640 (gdb+0x7f1bf7)
>      #13 operator() gdb/dwarf2/cooked-index.c:470 (gdb+0x7f0f40)
>      #14 _M_invoke /usr/include/c++/7/bits/std_function.h:316 (gdb+0x7f2909)
>      #15 std::function<void ()>::operator()() const /usr/include/c++/7/bits/std_function.h:706 (gdb+0x700952)
>      #16 void std::__invoke_impl<void, std::function<void ()>&>(std::__invoke_other, std::function<void ()>&) /usr/include/c++/7/bits/invoke.h:60 (gdb+0x7381a0)
>      #17 std::__invoke_result<std::function<void ()>&>::type std::__invoke<std::function<void ()>&>(std::function<void ()>&) /usr/include/c++/7/bits/invoke.h:95 (gdb+0x737e91)
>      #18 std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run()::{lambda()#1}::operator()() const /usr/include/c++/7/future:1421 (gdb+0x737b59)
>      #19 std::__future_base::_Task_setter<std::unique_ptr<std::__future_base::_Result<void>, std::__future_base::_Result_base::_Deleter>, std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run()::{lambda()#1}, void>::operator()() const /usr/include/c++/7/future:1362 (gdb+0x738660)
>      #20 std::_Function_handler<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> (), std::__future_base::_Task_setter<std::unique_ptr<std::__future_base::_Result<void>, std::__future_base::_Result_base::_Deleter>, std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run()::{lambda()#1}, void> >::_M_invoke(std::_Any_data const&) /usr/include/c++/7/bits/std_function.h:302 (gdb+0x73825c)
>      #21 std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>::operator()() const /usr/include/c++/7/bits/std_function.h:706 (gdb+0x733623)
>      #22 std::__future_base::_State_baseV2::_M_do_set(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*) /usr/include/c++/7/future:561 (gdb+0x732bdf)
>      #23 void std::__invoke_impl<void, void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::__invoke_memfun_deref, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) /usr/include/c++/7/bits/invoke.h:73 (gdb+0x734c4f)
>      #24 std::__invoke_result<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>::type std::__invoke<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) /usr/include/c++/7/bits/invoke.h:95 (gdb+0x733bc5)
>      #25 std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&)::{lambda()#1}::operator()() const /usr/include/c++/7/mutex:672 (gdb+0x73300d)
>      #26 std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&)::{lambda()#2}::operator()() const /usr/include/c++/7/mutex:677 (gdb+0x7330b2)
>      #27 std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&)::{lambda()#2}::_FUN() /usr/include/c++/7/mutex:677 (gdb+0x7330f2)
>      #28 pthread_once <null> (libtsan.so.0+0x4457c)
>      #29 __gthread_once /usr/include/c++/7/x86_64-suse-linux/bits/gthr-default.h:699 (gdb+0x72f5dd)
>      #30 void std::call_once<void (std::__future_base::_State_baseV2::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*>(std::once_flag&, void (std::__future_base::_State_baseV2::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*, bool*), std::__future_base::_State_baseV2*&&, std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>*&&, bool*&&) /usr/include/c++/7/mutex:684 (gdb+0x733224)
>      #31 std::__future_base::_State_baseV2::_M_set_result(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()>, bool) /usr/include/c++/7/future:401 (gdb+0x732852)
>      #32 std::__future_base::_Task_state<std::function<void ()>, std::allocator<int>, void ()>::_M_run() /usr/include/c++/7/future:1423 (gdb+0x737bef)
>      #33 std::packaged_task<void ()>::operator()() /usr/include/c++/7/future:1556 (gdb+0x1daca1c)
>      #34 gdb::thread_pool::thread_function() gdbsupport/thread-pool.cc:242 (gdb+0x1dac33e)
>      #35 void std::__invoke_impl<void, void (gdb::thread_pool::*)(), gdb::thread_pool*>(std::__invoke_memfun_deref, void (gdb::thread_pool::*&&)(), gdb::thread_pool*&&) /usr/include/c++/7/bits/invoke.h:73 (gdb+0x1dad3ed)
>      #36 std::__invoke_result<void (gdb::thread_pool::*)(), gdb::thread_pool*>::type std::__invoke<void (gdb::thread_pool::*)(), gdb::thread_pool*>(void (gdb::thread_pool::*&&)(), gdb::thread_pool*&&) /usr/include/c++/7/bits/invoke.h:95 (gdb+0x1dac81e)
>      #37 decltype (__invoke((_S_declval<0ul>)(), (_S_declval<1ul>)())) std::thread::_Invoker<std::tuple<void (gdb::thread_pool::*)(), gdb::thread_pool*> >::_M_invoke<0ul, 1ul>(std::_Index_tuple<0ul, 1ul>) /usr/include/c++/7/thread:234 (gdb+0x1dafb50)
>      #38 std::thread::_Invoker<std::tuple<void (gdb::thread_pool::*)(), gdb::thread_pool*> >::operator()() /usr/include/c++/7/thread:243 (gdb+0x1dafadb)
>      #39 std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (gdb::thread_pool::*)(), gdb::thread_pool*> > >::_M_run() /usr/include/c++/7/thread:186 (gdb+0x1dafa90)
>      #40 <null> <null> (libstdc++.so.6+0xdcac2)
> 
>    Previous write of size 4 at 0x7b4400097d08 by main thread:
>      #0 bfd_open_file bfd/cache.c:584 (gdb+0x148bc90)
>      #1 bfd_cache_lookup_worker bfd/cache.c:261 (gdb+0x148b228)
>      #2 cache_bseek bfd/cache.c:289 (gdb+0x148b422)
>      #3 bfd_seek bfd/bfdio.c:459 (gdb+0x1489d2f)
>      #4 _bfd_generic_get_section_contents bfd/libbfd.c:1069 (gdb+0x14978a2)
>      #5 bfd_get_section_contents bfd/section.c:1606 (gdb+0x149cd7a)
>      #6 gdb_bfd_scan_elf_dyntag(int, bfd*, unsigned long*, unsigned long*) gdb/solib.c:1601 (gdb+0xed8fc8)
>      #7 elf_locate_base gdb/solib-svr4.c:705 (gdb+0xec29aa)
>      #8 svr4_iterate_over_objfiles_in_search_order gdb/solib-svr4.c:3430 (gdb+0xeca65b)
>      #9 gdbarch_iterate_over_objfiles_in_search_order(gdbarch*, gdb::function_view<bool (objfile*)>, objfile*) gdb/gdbarch.c:5041 (gdb+0x537cad)
>      #10 find_main_name gdb/symtab.c:6270 (gdb+0xf744a3)
>      #11 main_language() gdb/symtab.c:6313 (gdb+0xf74597)
>      #12 set_initial_language() gdb/symfile.c:1700 (gdb+0xf4295a)
>      #13 symbol_file_add_main_1 gdb/symfile.c:1212 (gdb+0xf40f28)
>      #14 symbol_file_command(char const*, int) gdb/symfile.c:1681 (gdb+0xf428cf)
>      #15 file_command gdb/exec.c:554 (gdb+0x94f849)
>      #16 do_simple_func gdb/cli/cli-decode.c:95 (gdb+0x6d9528)
>      #17 cmd_func(cmd_list_element*, char const*, int) gdb/cli/cli-decode.c:2735 (gdb+0x6e0f69)
>      #18 execute_command(char const*, int) gdb/top.c:575 (gdb+0xff313a)
>      #19 command_handler(char const*) gdb/event-top.c:552 (gdb+0x94aedc)
>      #20 command_line_handler(std::unique_ptr<char, gdb::xfree_deleter<char> >&&) gdb/event-top.c:788 (gdb+0x94b599)
>      #21 tui_command_line_handler gdb/tui/tui-interp.c:104 (gdb+0x103489a)
>      #22 gdb_rl_callback_handler gdb/event-top.c:259 (gdb+0x94a481)
>      #23 rl_callback_read_char readline/readline/callback.c:290 (gdb+0x11bdf5b)
>      #24 gdb_rl_callback_read_char_wrapper_noexcept gdb/event-top.c:195 (gdb+0x94a280)
>      #25 gdb_rl_callback_read_char_wrapper gdb/event-top.c:234 (gdb+0x94a341)
>      #26 stdin_event_handler gdb/ui.c:155 (gdb+0x1074b3e)
>      #27 handle_file_event gdbsupport/event-loop.cc:573 (gdb+0x1d9536e)
>      #28 gdb_wait_for_event gdbsupport/event-loop.cc:694 (gdb+0x1d95aa6)
>      #29 gdb_do_one_event(int) gdbsupport/event-loop.cc:264 (gdb+0x1d93e92)
>      #30 start_event_loop gdb/main.c:412 (gdb+0xb5a472)
>      #31 captured_command_loop gdb/main.c:476 (gdb+0xb5a661)
>      #32 captured_main gdb/main.c:1320 (gdb+0xb5c7e1)
>      #33 gdb_main(captured_main_args*) gdb/main.c:1339 (gdb+0xb5c890)
>      #34 main gdb/gdb.c:32 (gdb+0x416776)
>    ...
> SUMMARY: ThreadSanitizer: data race bfd.h:2376 in bfd_get_section_limit_octets
> ...
> 
> Tom de Vries (3):
>    [gdb/symtab] Fix data race in index_cache::enable
>    [gdb/symtab] Fix data race in bfd_open_file
>    [gdb/symtab] Fix race on per_cu->queued
> 
>   gdb/dwarf2/cooked-index.c |  8 ++++--
>   gdb/dwarf2/cooked-index.h |  3 ++
>   gdb/dwarf2/index-cache.c  | 58 +++++++++++++++++++++++++++++----------
>   gdb/dwarf2/index-cache.h  | 20 +++++++++++++-
>   gdb/dwarf2/read.h         |  2 +-
>   5 files changed, 72 insertions(+), 19 deletions(-)
> 
> 
> base-commit: 29c108c9610640439daa5244a573348b7c47d994