diff options
authorRich Felker <>2020-10-30 11:21:06 -0400
committerRich Felker <>2020-10-30 11:21:06 -0400
commit27b2fc9d6db956359727a66c262f1e69995660aa (patch)
parent7c71792e87691451f2a6b76348e83ad1889f1dcb (diff)
fix missing-wake regression in pthread_cond_wait
the reasoning in commit 2d0bbe6c788938d1332609c014eeebc1dff966ac was not entirely correct. while it's true that setting the waiters flag ensures that the next unlock will perform a wake, it's possible that the wake is consumed by a mutex waiter that has no relationship with the condvar wait queue being processed, which then takes the mutex. when that thread subsequently unlocks, it sees no waiters, and leaves the rest of the condvar queue stuck. bring back the waiter count adjustment, but skip it for PI mutexes, for which a successful lock-after-waiting always sets the waiters bit. if future changes are made to bring this same waiters-bit contract to all lock types, this can be reverted.
1 files changed, 5 insertions, 0 deletions
diff --git a/src/thread/pthread_cond_timedwait.c b/src/thread/pthread_cond_timedwait.c
index f5f37af1..a0cd4904 100644
--- a/src/thread/pthread_cond_timedwait.c
+++ b/src/thread/pthread_cond_timedwait.c
@@ -146,12 +146,17 @@ relock:
if (oldstate == WAITING) goto done;
+ if (! && !(m->_m_type & 8))
+ a_inc(&m->_m_waiters);
/* Unlock the barrier that's holding back the next waiter, and
* either wake it or requeue it to the mutex. */
if (node.prev) {
int val = m->_m_lock;
if (val>0) a_cas(&m->_m_lock, val, val|0x80000000);
unlock_requeue(&node.prev->barrier, &m->_m_lock, m->_m_type & (8|128));
+ } else if (!!(m->_m_type & 8)) {
+ a_dec(&m->_m_waiters);
/* Since a signal was consumed, cancellation is not permitted. */