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new futex-requeue-based pthread_cond_broadcast implementation

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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.
This commit is contained in:
Rich Felker
2011-09-25 02:38:03 -04:00
parent 97c5b5a87c
commit cba4e1c0a3
4 changed files with 69 additions and 7 deletions
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41
src/thread/pthread_cond_broadcast.c
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@ -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;
}