further simplify and optimize new cond var

the main idea of the changes made is to have waiters wait directly on
the "barrier" lock that was used to prevent them from making forward
progress too early rather than first waiting on the atomic state value
and then attempting to lock the barrier.

in addition, adjustments to the mutex waiter count are optimized.
previously, each waking waiter decremented the count (unless it was
the first) then immediately incremented it again for the next waiter
(unless it was the last). this was a roundabout was of achieving the
equivalent of incrementing it once for the first waiter and
decrementing it once for the last.

src/thread/pthread_cond_timedwait.c

@@ -24,7 +24,7 @@
 
 struct waiter {
 	struct waiter *prev, *next;
-	int state, barrier, requeued, mutex_ret;
+	int state, barrier, mutex_ret;
 	int *notify;
 	pthread_mutex_t *mutex;
 	pthread_cond_t *cond;
@@ -48,6 +48,14 @@ static inline void unlock(volatile int *l)
 		__wake(l, 1, 1);
 }
 
+static inline void unlock_requeue(volatile int *l, volatile int *r, int w)
+{
+	a_store(l, 0);
+	if (w) __wake(l, 1, 1);
+	else __syscall(SYS_futex, l, FUTEX_REQUEUE|128, 0, 1, r) != -EINVAL
+		|| __syscall(SYS_futex, l, FUTEX_REQUEUE, 0, 1, r);
+}
+
 enum {
 	WAITING,
 	SIGNALED,
@@ -98,24 +106,16 @@ static void unwait(void *arg)
 
 	if (oldstate == WAITING) return;
 
-	/* If this thread was requeued to the mutex, undo the extra
+	if (!node->next) a_inc(&node->mutex->_m_waiters);
-	 * waiter count that was added to the mutex. */
-	if (node->requeued) a_dec(&node->mutex->_m_waiters);
 
-	/* Unlock the barrier that's holding back the next waiter,
+	/* Unlock the barrier that's holding back the next waiter, and
-	 * and either wake it or requeue it to the mutex. */
+	 * either wake it or requeue it to the mutex. */
 	if (node->prev) {
-		unlock(&node->prev->barrier);
+		unlock_requeue(&node->prev->barrier,
-		node->prev->requeued = 1;
+			&node->mutex->_m_lock,
-		a_inc(&node->mutex->_m_waiters);
+			node->mutex->_m_type & 128);
-		/* The futex requeue command cannot requeue from
+	} else {
-		 * private to shared, so for process-shared mutexes,
+		a_dec(&node->mutex->_m_waiters);
-		 * simply wake the target. */
-		int wake = node->mutex->_m_type & 128;
-		__syscall(SYS_futex, &node->prev->state, FUTEX_REQUEUE|128,
-			wake, 1, &node->mutex->_m_lock) != -EINVAL
-		|| __syscall(SYS_futex, &node->prev->state, FUTEX_REQUEUE,
-			0, 1, &node->mutex->_m_lock);
 	}
 }
 
@@ -140,9 +140,9 @@ int pthread_cond_timedwait(pthread_cond_t *restrict c, pthread_mutex_t *restrict
 	} else {
 		lock(&c->_c_lock);
 
-		node.barrier = 1;
+		seq = node.barrier = 2;
-		fut = &node.state;
+		fut = &node.barrier;
-		seq = node.state = WAITING;
+		node.state = WAITING;
 		node.next = c->_c_head;
 		c->_c_head = &node;
 		if (!c->_c_tail) c->_c_tail = &node;
@@ -169,11 +169,6 @@ int __private_cond_signal(pthread_cond_t *c, int n)
 
 	lock(&c->_c_lock);
 	for (p=c->_c_tail; n && p; p=p->prev) {
-		/* The per-waiter-node barrier lock is held at this
-		 * point, so while the following CAS may allow forward
-		 * progress in the target thread, it doesn't allow
-		 * access to the waiter list yet. Ideally the target
-		 * does not run until the futex wake anyway. */
 		if (a_cas(&p->state, WAITING, SIGNALED) != WAITING) {
 			ref++;
 			p->notify = &ref;
@@ -198,10 +193,7 @@ int __private_cond_signal(pthread_cond_t *c, int n)
 	while ((cur = ref)) __wait(&ref, 0, cur, 1);
 
 	/* Allow first signaled waiter, if any, to proceed. */
-	if (first) {
+	if (first) unlock(&first->barrier);
-		__wake(&first->state, 1, 1);
-		unlock(&first->barrier);
-	}
 
 	return 0;
 }