Yeah, that looks like a bug to me. We need to keep the check before the lock
but otherwise this is fine and should be fixed in 2.0.2.
-Nathan
> On Sep 21, 2016, at 3:16 AM, DEVEZE, PASCAL <pascal.dev...@atos.net> wrote:
>
> I encountered a deadlock in sync_wait_mt().
>
> After investigations, it appears that a first thread executing
> wait_sync_update() decrements sync->count just after a second thread in
> sync_wait_mt() made the test :
>
> if(sync->count <= 0)
> return (0 == sync->status) ? OPAL_SUCCESS : OPAL_ERROR;
>
> After that, there is a narrow window in which the first thread may call
> pthread_cond_signal() before the second thread calls pthread_cond_wait().
>
> If I protect this test by the sync->lock, this window is closed and the
> problem does not reproduce.
>
> To easy reproduce the problem, just add a call to usleep(100) before the call
> to pthread_mutex(&sync->lock);
>
> So my proposed patch is:
>
> diff --git a/opal/threads/wait_sync.c b/opal/threads/wait_sync.c
> index c9b9137..2f90965 100644
> --- a/opal/threads/wait_sync.c
> +++ b/opal/threads/wait_sync.c
> @@ -25,12 +25,14 @@ static ompi_wait_sync_t* wait_sync_list = NULL;
>
> int sync_wait_mt(ompi_wait_sync_t *sync)
> {
> - if(sync->count <= 0)
> - return (0 == sync->status) ? OPAL_SUCCESS : OPAL_ERROR;
> -
> /* lock so nobody can signal us during the list updating */
> pthread_mutex_lock(&sync->lock);
>
> + if(sync->count <= 0) {
> + pthread_mutex_unlock(&sync->lock);
> + return (0 == sync->status) ? OPAL_SUCCESS : OPAL_ERROR;
> + }
> +
> /* Insert sync on the list of pending synchronization constructs */
> OPAL_THREAD_LOCK(&wait_sync_lock);
> if( NULL == wait_sync_list ) {
>
> For performance reasons, it is also possible to leave the first test call. So
> if the request is terminated, we do not spend time to take and free the lock.
>
>
>
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