new futex-requeue-based pthread_cond_broadcast implementation

this avoids the "stampede effect" where pthread_cond_broadcast would
result in all waiters waking up simultaneously, only to immediately
contend for the mutex and go back to sleep.

1 files changed, 19 insertions, 2 deletions

diff --git a/src/thread/pthread_cond_timedwait.c b/src/thread/pthread_cond_timedwait.c
index c71edc95..9616dd85 100644
--- a/src/thread/pthread_cond_timedwait.c
+++ b/src/thread/pthread_cond_timedwait.c
@@ -5,10 +5,25 @@ struct cm {
 	pthread_mutex_t *m;
 };
 
+static void unwait(pthread_cond_t *c, pthread_mutex_t *m)
+{
+	int w;
+
+	/* Cannot leave waiting status if there are any live broadcasters
+	 * which might be inspecting/using the mutex. */
+	while ((w=c->_c_bcast)) __wait(&c->_c_bcast, &c->_c_leavers, w, 0);
+
+	/* If the waiter count is zero, it must be the case that the
+	 * caller's count has been moved to the mutex due to bcast. */
+	do w = c->_c_waiters;
+	while (w && a_cas(&c->_c_waiters, w, w-1)!=w);
+	if (!w) a_dec(&m->_m_waiters);
+}
+
 static void cleanup(void *p)
 {
 	struct cm *cm = p;
-	a_dec(&cm->c->_c_waiters);
+	unwait(cm->c, cm->m);
 	pthread_mutex_lock(cm->m);
 }
 
@@ -22,6 +37,8 @@ int pthread_cond_timedwait(pthread_cond_t *c, pthread_mutex_t *m, const struct t
 
 	pthread_testcancel();
 
+	if (c->_c_mutex != (void *)-1) c->_c_mutex = m;
+
 	a_inc(&c->_c_waiters);
 	c->_c_block = tid = pthread_self()->tid;
 
@@ -31,7 +48,7 @@ int pthread_cond_timedwait(pthread_cond_t *c, pthread_mutex_t *m, const struct t
 	while (c->_c_block == tid && (!e || e==EINTR));
 	if (e == EINTR) e = 0;
 
-	a_dec(&c->_c_waiters);
+	unwait(c, m);
 
 	if ((r=pthread_mutex_lock(m))) return r;