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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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21
src/thread/pthread_cond_timedwait.c
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@ -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;