From c11d1e696723f41d7873332e51fb6858b417fa5f Mon Sep 17 00:00:00 2001 From: Rich Felker Date: Sun, 25 Sep 2011 21:14:40 -0400 Subject: [PATCH] revert previous change in cond var waiter move using swap has a race condition: the waiters must be added to the mutex waiter count *before* they are taken off the cond var waiter count, or wake events can be lost. --- src/thread/pthread_cond_broadcast.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/src/thread/pthread_cond_broadcast.c b/src/thread/pthread_cond_broadcast.c index 4fab9994..7e5ea91c 100644 --- a/src/thread/pthread_cond_broadcast.c +++ b/src/thread/pthread_cond_broadcast.c @@ -30,8 +30,12 @@ int pthread_cond_broadcast(pthread_cond_t *c) } /* Move waiter count to the mutex */ - w = a_swap(&c->_c_waiters, 0); - a_fetch_add(&m->_m_waiters, w); + for (;;) { + w = c->_c_waiters; + a_fetch_add(&m->_m_waiters, w); + if (a_cas(&c->_c_waiters, w, 0) == w) break; + a_fetch_add(&m->_m_waiters, -w); + } /* Perform the futex requeue, waking one waiter unless we know * that the calling thread holds the mutex. */ -- 2.25.1