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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
commit 1fa0521010
3 changed files with 17 additions and 14 deletions
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22
src/thread/pthread_cond_timedwait.c
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@ -7,6 +7,8 @@ struct cm {
static void unwait(pthread_cond_t *c, pthread_mutex_t *m)
{
int w;
/* Removing a waiter is non-trivial if we could be using requeue
* based broadcast signals, due to mutex access issues, etc. */
@ -18,8 +20,10 @@ static void unwait(pthread_cond_t *c, pthread_mutex_t *m)
while (a_swap(&c->_c_lock, 1))
__wait(&c->_c_lock, &c->_c_lockwait, 1, 1);
if (c->_c_waiters) c->_c_waiters--;
else a_dec(&m->_m_waiters);
/* Atomically decrement waiters2 if positive, else mutex waiters. */
do w = c->_c_waiters2;
while (w && a_cas(&c->_c_waiters2, w, w-1)!=w);
if (!w) a_dec(&m->_m_waiters);
a_store(&c->_c_lock, 0);
if (c->_c_lockwait) __wake(&c->_c_lock, 1, 1);
@ -42,16 +46,10 @@ int pthread_cond_timedwait(pthread_cond_t *c, pthread_mutex_t *m, const struct t
pthread_testcancel();
if (c->_c_mutex == (void *)-1) {
a_inc(&c->_c_waiters);
} else {
c->_c_mutex = m;
while (a_swap(&c->_c_lock, 1))
__wait(&c->_c_lock, &c->_c_lockwait, 1, 1);
c->_c_waiters++;
a_store(&c->_c_lock, 0);
if (c->_c_lockwait) __wake(&c->_c_lock, 1, 1);
}
if (c->_c_mutex != (void *)-1) c->_c_mutex = m;
a_inc(&c->_c_waiters);
a_inc(&c->_c_waiters2);
seq = c->_c_seq;