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_broadcast.c | 41 ++++++++++++++++++++++++++++++++++++- 1 file changed, 40 insertions(+), 1 deletion(-) (limited to 'src/thread/pthread_cond_broadcast.c') 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; } -- cgit v1.2.1