[RFC,1/8] gdb, infrun: do not discard a step-completed pending waitstatus
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Consider a scenario with two breakpoints on adjacent instructions:
bp1 at 0xf00
bp2 at 0xf01
as well as two threads in all-stop-on-top-of-non-stop mode.
Assume that threads A hits bp1 and we report the breakpoint hit to the
user. When the user continues, we start a step-over for thread A at 0xf00.
Assume that thread B now hits bp1 and we report the breakpoint hit to the
user. We stop all threads to report the event. Meanwhile, the step-over
of thread A completes, so we save the pending waitstatus (stop_pc=0xf01,
currently_stepping=1) of thread A.
When the user continues, clear_proceed_status_thread() discards the
pending step completed waitstatus of thread A, and proceed() starts
another step-over for thread A at 0xf01.
We skip bp2 for thread A.
Remove the code in clear_proceed_status_thread() that discards a step
completed waitstatus and let it get handled normally. This involves
calling finish_step_over() to clean up displaced stepping state.
This introduces an additional step completed stop event for thread A that
results in two user-visible changes in behavior when switching to thread A
and resuming that thread:
- stepi/nexti completes without making progress
- adding a breakpoint at the current location before resuming hits that
breakpoint
Fixes gdb/34380.
Tested-By: Guinevere Larsen <guinevere@redhat.com>
---
gdb/infrun.c | 25 ++-----
gdb/testsuite/gdb.threads/adjacent-bp.c | 48 +++++++++++++
gdb/testsuite/gdb.threads/adjacent-bp.exp | 86 +++++++++++++++++++++++
3 files changed, 139 insertions(+), 20 deletions(-)
create mode 100644 gdb/testsuite/gdb.threads/adjacent-bp.c
create mode 100644 gdb/testsuite/gdb.threads/adjacent-bp.exp
@@ -3100,28 +3100,13 @@ clear_proceed_status_thread (struct thread_info *tp)
infrun_debug_printf ("%s", tp->ptid.to_string ().c_str ());
gdb_assert (tp->internal_state () != THREAD_INT_RUNNING);
- /* If we're starting a new sequence, then the previous finished
- single-step is no longer relevant. */
if (tp->has_pending_waitstatus ())
{
- if (tp->stop_reason () == TARGET_STOPPED_BY_SINGLE_STEP)
- {
- infrun_debug_printf ("pending event of %s was a finished step. "
- "Discarding.",
- tp->ptid.to_string ().c_str ());
-
- tp->set_internal_state (THREAD_INT_STOPPED);
- tp->clear_pending_waitstatus ();
- tp->set_stop_reason (TARGET_STOPPED_BY_NO_REASON);
- }
- else
- {
- infrun_debug_printf
- ("thread %s has pending wait status %s (currently_stepping=%d).",
- tp->ptid.to_string ().c_str (),
- tp->pending_waitstatus ().to_string ().c_str (),
- tp->control.currently_stepping);
- }
+ infrun_debug_printf
+ ("thread %s has pending wait status %s (currently_stepping=%d).",
+ tp->ptid.to_string ().c_str (),
+ tp->pending_waitstatus ().to_string ().c_str (),
+ tp->control.currently_stepping);
}
/* If this signal should not be seen by program, give it zero.
new file mode 100644
@@ -0,0 +1,48 @@
+/* Copyright 2026 Free Software Foundation, Inc.
+
+ This file is part of GDB.
+
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>. */
+
+#include <pthread.h>
+#include <unistd.h>
+
+static pthread_barrier_t barrier;
+
+static void *
+test (void *arg)
+{
+ pthread_barrier_wait (&barrier);
+ int a = 0; /* break here. */
+ int b = 0;
+ int c = 0;
+ return arg;
+}
+
+int
+main ()
+{
+ pthread_t th;
+
+ alarm (500);
+
+ pthread_barrier_init (&barrier, NULL, 2);
+ pthread_create (&th, NULL, test, NULL);
+ test (NULL);
+
+ pthread_join (th, NULL);
+ pthread_barrier_destroy (&barrier);
+
+ return 0;
+}
new file mode 100644
@@ -0,0 +1,86 @@
+# Copyright 2026 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+# Test that GDB does not skip a breakpoint when a step-over completes
+# while another event leads to a stop.
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" ${testfile} ${srcfile} \
+ {debug pthreads}]} {
+ return
+}
+
+if {![runto_main]} {
+ return
+}
+
+# Find a sequence of adjacent instructions.
+set bp_line [gdb_get_line_number "break here"]
+set pcs {}
+gdb_test_multiple "info line $bp_line" "" {
+ -re -wrap "starts at address ($hex).*" {
+ pass $gdb_test_name
+
+ set line "\\s+($hex) \[^\r\n\]+"
+ gdb_test_multiple "x/3i $expect_out(1,string)" "disassemble" {
+ -re -wrap "$line\r\n$line\r\n$line.*" {
+ pass $gdb_test_name
+
+ lappend pcs $expect_out(1,string)
+ lappend pcs $expect_out(2,string)
+ lappend pcs $expect_out(3,string)
+ }
+ -re -wrap "" {
+ fail $gdb_test_name
+ }
+ }
+ }
+ -re -wrap "" {
+ fail $gdb_test_name
+ }
+}
+
+# Set breakpoints on adjacent instructions.
+foreach pc $pcs {
+ gdb_breakpoint "\*$pc"
+}
+
+# Continue from breakpoint to breakpoint.
+set hits [dict create]
+set iter 0
+gdb_test_multiple "continue" "" {
+ -re -wrap "hit Breakpoint.*" {
+ dict incr hits [get_hexadecimal_valueof "\$pc" invalid "stop $iter"]
+ incr iter
+ send_gdb "continue\n"
+ exp_continue
+ }
+ -re -wrap "$inferior_exited_re normally.*" {
+ pass "$gdb_test_name"
+ }
+}
+
+# We expect all breakpoints to be hit by both threads.
+foreach pc $pcs {
+ if {[dict exists $hits $pc]} {
+ gdb_assert {[dict get $hits $pc] eq 2} "breakpoint at $pc"
+ dict unset hits $pc
+ } else {
+ fail "breakpoint at $pc"
+ }
+}
+# And no unrelated stops.
+gdb_assert {[dict size $hits] eq 0} "no extra stops"