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fix lost signals in cond vars

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due to moving waiters from the cond var to the mutex in bcast, these
waiters upon wakeup would steal slots in the count from newer waiters
that had not yet been signaled, preventing the signal function from
taking any action.

to solve the problem, we simply use two separate waiter counts, and so
that the original "total" waiters count is undisturbed by broadcast
and still available for signal.
This commit is contained in:
Rich Felker
2011-09-26 12:54:28 -04:00
parent fd142e5ec4
commit 1fa0521010
3 changed files with 17 additions and 14 deletions
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8
src/thread/pthread_cond_broadcast.c
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@@ -22,8 +22,12 @@ int pthread_cond_broadcast(pthread_cond_t *c)
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);
for (;;) {
int w = c->_c_waiters2;
a_fetch_add(&m->_m_waiters, w);
if (a_cas(&c->_c_waiters2, w, 0) == w) break;
a_fetch_add(&m->_m_waiters, -w);
}
/* Perform the futex requeue, waking one waiter unless we know
* that the calling thread holds the mutex. */