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.

diff --git a/src/thread/pthread_cond_broadcast.c b/src/thread/pthread_cond_broadcast.c
index 4fab999..7e5ea91 100644 (file)
--- 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. */