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.

3 files changed, 63 insertions, 4 deletions

diff --git a/src/thread/pthread_cond_broadcast.c b/src/thread/pthread_cond_broadcast.c
index dec91164..3daa7817 100644
--- a/src/thread/pthread_cond_broadcast.c
+++ b/src/thread/pthread_cond_broadcast.c
@@ -1,8 +1,47 @@
 #include "pthread_impl.h"
 
+static void unlock(pthread_cond_t *c)
+{
+	a_dec(&c->_c_bcast);
+	if (c->_c_leavers) __wake(&c->_c_bcast, -1, 0);
+}
+
 int pthread_cond_broadcast(pthread_cond_t *c)
 {
+	pthread_mutex_t *m;
+	int w;
+
+	if (!c->_c_waiters) return 0;
+	a_inc(&c->_c_bcast);
+	if (!c->_c_waiters) {
+		unlock(c);
+		return 0;
+	}
+
 	a_store(&c->_c_block, 0);
-	if (c->_c_waiters) __wake(&c->_c_block, -1, 0);
+
+	m = c->_c_mutex;
+
+	/* If mutex ptr is not available, simply wake all waiters. */
+	if (m == (void *)-1) {
+		unlock(c);
+		__wake(&c->_c_block, -1, 0);
+		return 0;
+	}
+
+	/* Move waiter count to the mutex */
+	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 if and only if
+	 * the calling thread does not hold the mutex. */
+	__syscall(SYS_futex, &c->_c_block, FUTEX_REQUEUE,
+		m->_m_lock!=pthread_self()->tid, INT_MAX, &m->_m_lock);
+
+	unlock(c);
 	return 0;
 }
diff --git a/src/thread/pthread_cond_init.c b/src/thread/pthread_cond_init.c
index dc4fb981..03aff768 100644
--- a/src/thread/pthread_cond_init.c
+++ b/src/thread/pthread_cond_init.c
@@ -3,6 +3,9 @@
 int pthread_cond_init(pthread_cond_t *c, const pthread_condattr_t *a)
 {
 	memset(c, 0, sizeof *c);
-	if (a) c->_c_clock = *a & 0x7fffffff;
+	if (a) {
+		c->_c_clock = *a & 0x7fffffff;
+		if (*a>>31) c->_c_mutex = (void *)-1;
+	}
 	return 0;
 }
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;