From cba4e1c0a3423ed07ef5c79f6e29360996f32fd7 Mon Sep 17 00:00:00 2001 From: Rich Felker Date: Sun, 25 Sep 2011 02:38:03 -0400 Subject: 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. --- src/thread/pthread_cond_timedwait.c | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-) (limited to 'src/thread/pthread_cond_timedwait.c') 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; -- cgit v1.2.1