[PATCH] gdb/testsuite: fix race condition in gdb.replay/missing-thread.c

simon.marchi
2026-02-06T17:58:42Z

From: Simon Marchi <simon.marchi@polymtl.ca>

I get frequent failures in gdb.replay/missing-thread.exp, starting with:

FAIL: gdb.replay/missing-thread.exp: non_stop=on: record_initial_logfile: continue (timeout)

I tracked this down to a race condition in the test program, causing it
not to generate the expected SIGTRAP. This can be reproduced by adding
a sleep(1) just after the pthread_create.

The expected behavior is:

- The main thread starts a second thread
- The main thread blocks on pthread_cond_wait, waiting to be notified
by the second thread (to make sure the second thread had time to
start)
- The second thread notifies the main thread using pthread_cond_signal
- The main thread sends a SIGTRAP to the second thread

However, this can happen:

- The main thread starts a second thread
- The second thread calls pthread_cond_signal, while no one is waiting
to the condvar
- The main thread blocks on pthread_cond_wait forever

Fix it by introducing a separate boolean predicate protected by the
shared mutex, and looping until it is true. This is the way
pthread_cond_wait is meant to be used. From pthread_cond_wait(3):

When using condition variables there is always a boolean predicate
involving shared variables associated with each condition wait that
is true if the thread should proceed. Spurious wakeups from the
pthread_cond_wait() or pthread_cond_timedwait() functions may occur.
Since the return from pthread_cond_wait() or
pthread_cond_timedwait() does not imply anything about the value of
this predicate, the predicate should be re-evaluated upon such
return.

Finally, make things static, just out of principle, and fix minor
formatting issues.

Change-Id: I62ba2085a2d506dc3d91e32cd5de48c43a3ff55e
---
gdb/testsuite/gdb.replay/missing-thread.c | 18 ++++++++++++------
1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/gdb/testsuite/gdb.replay/missing-thread.c b/gdb/testsuite/gdb.replay/missing-thread.c
index 0a17be1c7dd1..aad6396f24c2 100644
--- a/gdb/testsuite/gdb.replay/missing-thread.c
+++ b/gdb/testsuite/gdb.replay/missing-thread.c
@@ -22,23 +22,28 @@
#include <unistd.h>
#include <stdbool.h>

-pthread_mutex_t g_mutex = PTHREAD_MUTEX_INITIALIZER;
-pthread_cond_t g_condvar = PTHREAD_COND_INITIALIZER;
+static pthread_mutex_t g_mutex = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t g_condvar = PTHREAD_COND_INITIALIZER;
+static bool g_ready = false;

-void *
+static void *
worker_function (void *arg)
{
printf ("In worker, about to notify\n");
+
+ pthread_mutex_lock (&g_mutex);
+ g_ready = true;
+ pthread_mutex_unlock (&g_mutex);
pthread_cond_signal (&g_condvar);

while (true)
- sleep(1);
+ sleep (1);

return NULL;
}

int
-main()
+main ()
{
pthread_t my_thread;

@@ -46,7 +51,8 @@ main()
assert (result == 0);

pthread_mutex_lock (&g_mutex);
- pthread_cond_wait (&g_condvar, &g_mutex);
+ while (!g_ready)
+ pthread_cond_wait (&g_condvar, &g_mutex);

printf ("In main, have been woken.\n");
pthread_mutex_unlock (&g_mutex);

base-commit: deb47060b5812c6c41ecc790cb28898fcbc45c93
--
2.53.0