fix lost signals in cond vars

due to moving waiters from the cond var to the mutex in bcast, these
waiters upon wakeup would steal slots in the count from newer waiters
that had not yet been signaled, preventing the signal function from
taking any action.

to solve the problem, we simply use two separate waiter counts, and so
that the original "total" waiters count is undisturbed by broadcast
and still available for signal.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/thread/pthread_cond_broadcast.c b/src/thread/pthread_cond_broadcast.c
index bf6de048..9c6a462b 100644
--- a/src/thread/pthread_cond_broadcast.c
+++ b/src/thread/pthread_cond_broadcast.c
@@ -22,8 +22,12 @@ int pthread_cond_broadcast(pthread_cond_t *c)
 	m = c->_c_mutex;
 
 	/* Move waiter count to the mutex */
-	a_fetch_add(&m->_m_waiters, c->_c_waiters);
-	a_store(&c->_c_waiters, 0);
+	for (;;) {
+		int w = c->_c_waiters2;
+		a_fetch_add(&m->_m_waiters, w);
+		if (a_cas(&c->_c_waiters2, 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. */