mirror of
https://github.com/fluencelabs/musl
synced 2025-06-10 13:31:37 +00:00
729d6368bd
testing revealed that the old implementation, while correct, was giving way too many spurious wakeups due to races changing the value of the condition futex. in a test program with 5 threads receiving broadcast signals, the number of returns from pthread_cond_wait was roughly 3 times what it should have been (2 spurious wakeups for every legitimate wakeup). moreover, the magnitude of this effect seems to grow with the number of threads. the old implementation may also have had some nasty race conditions with reuse of the cond var with a new mutex. the new implementation is based on incrementing a sequence number with each signal event. this sequence number has nothing to do with the number of threads intended to be woken; it's only used to provide a value for the futex wait to avoid deadlock. in theory there is a danger of race conditions due to the value wrapping around after 2^32 signals. it would be nice to eliminate that, if there's a way. testing showed no spurious wakeups (though they are of course possible) with the new implementation, as well as slightly improved performance.
40 lines
936 B
C
40 lines
936 B
C
#include "pthread_impl.h"
|
|
|
|
int pthread_cond_broadcast(pthread_cond_t *c)
|
|
{
|
|
pthread_mutex_t *m;
|
|
|
|
if (!c->_c_waiters) return 0;
|
|
|
|
a_inc(&c->_c_seq);
|
|
|
|
/* If cond var is process-shared, simply wake all waiters. */
|
|
if (c->_c_mutex == (void *)-1) {
|
|
__wake(&c->_c_seq, -1, 0);
|
|
return 0;
|
|
}
|
|
|
|
/* Block waiters from returning so we can use the mutex. */
|
|
while (a_swap(&c->_c_lock, 1))
|
|
__wait(&c->_c_lock, &c->_c_lockwait, 1, 1);
|
|
if (!c->_c_waiters)
|
|
goto out;
|
|
m = c->_c_mutex;
|
|
|
|
/* Move waiter count to the mutex */
|
|
a_fetch_add(&m->_m_waiters, c->_c_waiters);
|
|
a_store(&c->_c_waiters, 0);
|
|
|
|
/* Perform the futex requeue, waking one waiter unless we know
|
|
* that the calling thread holds the mutex. */
|
|
__syscall(SYS_futex, &c->_c_seq, FUTEX_REQUEUE,
|
|
!m->_m_type || (m->_m_lock&INT_MAX)!=pthread_self()->tid,
|
|
INT_MAX, &m->_m_lock);
|
|
|
|
out:
|
|
a_store(&c->_c_lock, 0);
|
|
if (c->_c_lockwait) __wake(&c->_c_lock, 1, 0);
|
|
|
|
return 0;
|
|
}
|