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

Simon Marchi
2026-02-20T15:24:36Z

Ping.

On 2/6/26 12:57 PM, simon.marchi@polymtl.ca wrote:
> 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
>