[PATCH v3] gdb/infrun: do not restart a stepped thread if not running

Tankut Baris Aktemur
2026-02-23T10:39:27Z

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======
Changes in V3:

* Addressed Kevin's comments from
https://sourceware.org/pipermail/gdb-patches/2026-February/225235.html

* Rebased on the current master; ran regression tests again.

Changes in V2:

* Changed the thread state check to `(tp->state != THREAD_RUNNING)`.
* Rebased on the current master.

The latest discussion was
https://sourceware.org/pipermail/gdb-patches/2023-May/199613.html

======

Suppose we have two inferiors on an all-stop target with schedule-multi
set on:

$ gdb -q
(gdb) target extended-remote | gdbserver --multi -
Remote debugging using | gdbserver --multi -
Remote debugging using stdio
(gdb) file /temp/test
Reading symbols from /temp/test...
(gdb) set remote exec-file /temp/test
(gdb) start
Temporary breakpoint 1 at 0x115c: file test.c, line 8.
Starting program: /temp/test
stdin/stdout redirected
Process /temp/test created; pid = 864027
...

Temporary breakpoint 1, main (argc=1, argv=0x7fffffffd218) at test.c:8
8 foo();
(gdb) add-inferior
[New inferior 2]
Added inferior 2 on connection 1 (extended-remote | gdbserver --multi -)
(gdb) inferior 2
[Switching to inferior 2 [<null>] (<noexec>)]
(gdb) file /temp/test
Reading symbols from /temp/test...
(gdb) set remote exec-file /temp/test
(gdb) tbreak 2
Temporary breakpoint 2 at 0x555555555131: /temp/test.c:2. (2 locations)
(gdb) run
Starting program: /temp/test
stdin/stdout redirected
Process /temp/test created; pid = 864430
...

Thread 2.1 "test" hit Temporary breakpoint 2, foo () at test.c:2
2 int a = 42;
(gdb) set schedule-multi on
(gdb)

At this point, detaching the first inferior works fine:

(gdb) detach inferiors 1
Detaching from program: /temp/test, process 858904
Detaching from process 858904
[Inferior 1 (process 858904) detached]
(gdb) info inferiors
Num Description Connection Executable
1 <null> 1 (extended-remote | gdbserver --multi -) /temp/test
* 2 process 858925 1 (extended-remote | gdbserver --multi -) /temp/test
(gdb)

Let us now repeat exactly the same scenario, but before detaching, we
make the current thread single-step an instruction:

...
Thread 2.1 "test" hit Temporary breakpoint 2, foo () at test.c:2
2 int a = 42;
(gdb) stepi
3 int b = 43;
(gdb) detach inferiors 1
Detaching from program: /temp/test, process 876580
Detaching from process 876580
gdbserver: Couldn't reap LWP 876580 while detaching: No child processes
[Inferior 1 (process 876580) detached]
(gdb) [Switching to Thread 877351.877351]
3 int b = 43;

There is a mysterious line info output. Running the scenario with
infrun debug logs reveals more information.

...
Thread 2.1 "test" hit Temporary breakpoint 2, foo () at test.c:2
2 int a = 42;
(gdb) stepi
3 int b = 43;
(gdb) set debug infrun on
(gdb) detach inferiors 1
[infrun] scoped_disable_commit_resumed: reason=detaching
[infrun] scoped_disable_commit_resumed: reason=detaching
Detaching from program: /temp/test, process 3537498
Detaching from process 3537498
gdbserver: Couldn't reap LWP 3537498 while detaching: No child processes
[Inferior 1 (process 3537498) detached]
[infrun] reset: reason=detaching
[infrun] start_step_over: enter
[infrun] start_step_over: stealing global queue of threads to step, length = 0
[infrun] operator(): step-over queue now empty
[infrun] start_step_over: exit
[infrun] restart_stepped_thread: switching back to stepped thread (stepping)
[infrun] keep_going_stepped_thread: resuming previously stepped thread
[infrun] keep_going_stepped_thread: expected thread advanced also (0x555555555131 -> 0x555555555138)
[infrun] clear_step_over_info: clearing step over info
[infrun] do_target_resume: resume_ptid=-1.0.0, step=0, sig=GDB_SIGNAL_0
[infrun] infrun_async: enable=1
[infrun] reset: reason=detaching
[infrun] maybe_set_commit_resumed_all_targets: enabling commit-resumed for target extended-remote
[infrun] maybe_call_commit_resumed_all_targets: calling commit_resumed for target extended-remote
(gdb) [infrun] fetch_inferior_event: enter
[infrun] scoped_disable_commit_resumed: reason=handling event
[infrun] do_target_wait: Found 2 inferiors, starting at #0
[infrun] random_pending_event_thread: None found.
[infrun] print_target_wait_results: target_wait (-1.0.0 [process -1], status) =
[infrun] print_target_wait_results: 3537875.3537875.0 [Thread 3537875.3537875],
[infrun] print_target_wait_results: status->kind = STOPPED, sig = GDB_SIGNAL_TRAP
[infrun] handle_inferior_event: status->kind = STOPPED, sig = GDB_SIGNAL_TRAP
[infrun] context_switch: Switching context from 0.0.0 to 3537875.3537875.0
[infrun] handle_signal_stop: stop_pc=0x555555555138
[infrun] handle_signal_stop: [3537875.3537875.0] hit its single-step breakpoint
[infrun] handle_signal_stop: delayed software breakpoint trap, ignoring
[infrun] process_event_stop_test: stepi/nexti
[infrun] stop_waiting: stop_waiting
[Switching to Thread 3537875.3537875]
3 }
[infrun] infrun_async: enable=0
[infrun] reset: reason=handling event
[infrun] maybe_set_commit_resumed_all_targets: not requesting commit-resumed for target extended-remote, no resumed threads
[infrun] fetch_inferior_event: exit

GDB attempted to do a step-over for the current thread. This takes us
to the commit that introduced restarting step-overs:

commit 408f66864a1a823591b26420410c982174c239a2
Author: Pedro Alves <pedro@palves.net>
Date: Mon Jan 11 20:01:58 2021 +0000

detach in all-stop with threads running

A following patch will add a testcase that has a number of threads
constantly stepping over a breakpoint, and then has GDB detach the
process, while threads are running. If we have more than one inferior
running, and we detach from just one of the inferiors, we expect that
the remaining inferior continues running. However, in all-stop, if
GDB needs to pause the target for the detach, nothing is re-resuming
the other inferiors after the detach. "info threads" shows the
threads as running, but they really aren't. This fixes it.

However, the thread that was resumed for step-over in our scenario did
not have an interrupted step-over; it had completed its stepi already.
More debugging reveals that the thread is resumed because of the
following two conditions in `restart_stepped_thread`:

if (tp->control.trap_expected)
{
infrun_debug_printf ("switching back to stepped thread (step-over)");

if (keep_going_stepped_thread (tp))
return true;
}

and

if (tp->control.step_range_end)
{
infrun_debug_printf ("switching back to stepped thread (stepping)");

if (keep_going_stepped_thread (tp))
return true;
}

The root cause of the problem is, `restart_stepped_thread` checks for
the thread state as

if (tp->state == THREAD_EXITED)
continue;

but the thread's state is THREAD_STOPPED. To fix, we change the state
check to

if (tp->state != THREAD_RUNNING)

Additionally, the 'trap_expected' and the 'step_range_end' fields of
the thread's control remain set even after the "stepi" command
completes, creating a half-baked internal state that can be misleading
when debugging. We address this problem by clearing the control
fields when stepping completes. We also add a regression test.

Regression-tested on X86_64 Linux using the default, native-gdbserver,
and native-extended-gdbserver board files.

Tested-By: Guinevere Larsen <guinevere@redhat.com>
---
gdb/infrun.c | 7 ++-
gdb/testsuite/gdb.multi/detach-stepi.c | 30 +++++++++++
gdb/testsuite/gdb.multi/detach-stepi.exp | 69 ++++++++++++++++++++++++
3 files changed, 104 insertions(+), 2 deletions(-)
create mode 100644 gdb/testsuite/gdb.multi/detach-stepi.c
create mode 100644 gdb/testsuite/gdb.multi/detach-stepi.exp

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 6bcd8ec5cc0..ee5551cb483 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -8459,7 +8459,7 @@ restart_stepped_thread (process_stratum_target *resume_target,

for (thread_info &tp : all_threads_safe ())
{
- if (tp.state == THREAD_EXITED)
+ if (tp.state != THREAD_RUNNING)
continue;

if (tp.has_pending_waitstatus ())
@@ -8483,7 +8483,7 @@ restart_stepped_thread (process_stratum_target *resume_target,

for (thread_info &tp : all_threads_safe ())
{
- if (tp.state == THREAD_EXITED)
+ if (tp.state != THREAD_RUNNING)
continue;

if (tp.has_pending_waitstatus ())
@@ -9204,6 +9204,9 @@ static void
end_stepping_range (struct execution_control_state *ecs)
{
ecs->event_thread->control.stop_step = 1;
+ ecs->event_thread->control.trap_expected = 0;
+ ecs->event_thread->control.step_range_start = 0;
+ ecs->event_thread->control.step_range_end = 0;
stop_waiting (ecs);
}

diff --git a/gdb/testsuite/gdb.multi/detach-stepi.c b/gdb/testsuite/gdb.multi/detach-stepi.c
new file mode 100644
index 00000000000..45d8d0c14b4
--- /dev/null
+++ b/gdb/testsuite/gdb.multi/detach-stepi.c
@@ -0,0 +1,30 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+ Copyright 2022-2026 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/>. */
+
+void
+a_function (void)
+{
+ int a = 42;
+}
+
+int
+main (void)
+{
+ int b = 43;
+ a_function ();
+ return 0;
+}
diff --git a/gdb/testsuite/gdb.multi/detach-stepi.exp b/gdb/testsuite/gdb.multi/detach-stepi.exp
new file mode 100644
index 00000000000..74beb022bfd
--- /dev/null
+++ b/gdb/testsuite/gdb.multi/detach-stepi.exp
@@ -0,0 +1,69 @@
+# This testcase is part of GDB, the GNU debugger.
+
+# Copyright 2022-2026 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 detaching from an inferior after a thread in another inferior
+# completes a stepi. This is a regression test for a bug that was
+# causing an inadvertent resume of the just-stepped thread.
+
+standard_testfile
+
+require allow_multi_inferior_tests
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile]} {
+ return -1
+}
+
+if {![runto_main]} {
+ return -1
+}
+
+delete_breakpoints
+
+# Set up inferior 2.
+gdb_test "add-inferior" "Added inferior .*" \
+ "add another inferior"
+gdb_test "inferior 2" "Switching to inferior .*" \
+ "switch inferior"
+
+gdb_load $binfile
+if {![runto "a_function"]} {
+ return -1
+}
+
+gdb_test "info inferiors" ".*" "info inferiors for logging"
+
+# The bug for which this regression test is written appears in
+# schedule-multi mode.
+gdb_test_no_output "set schedule-multiple on"
+
+# Single-step the thread in Inferior 2, then detach Inferior 1.
+gdb_test "info threads" ".*" "threads before stepi"
+gdb_test "stepi" ".*"
+gdb_test "info threads" ".*" "threads after stepi"
+
+gdb_test "detach inferior 1" "Inferior 1 \[^\r\n\]+ detached\\]"
+
+# Bad GDB did an unexpected switch to a thread after the detach
+# command above.
+gdb_test_multiple "print 123" "no unexpected switch" {
+ -re "Switching to .*" {
+ fail $gdb_test_name
+ }
+ -re -wrap "" {
+ pass $gdb_test_name
+ }
+}
--
2.34.1

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