refrain from spinning on locks when there is already a waiter

if there is already a waiter for a lock, spinning on the lock is
essentially an attempt to steal it from whichever waiter would obtain
it via any priority rules in place, and is therefore undesirable. in
the current implementation, there is always an inherent race window at
unlock during which a newly-arriving thread may steal the lock from
the existing waiters, but we should aim to keep this window minimal
rather than enlarging it.

src/thread/__wait.c

@@ -4,7 +4,7 @@ void __wait(volatile int *addr, volatile int *waiters, int val, int priv)
 {
 	int spins=100;
 	if (priv) priv = 128;
-	while (spins--) {
+	while (spins-- && (!waiters || !*waiters)) {
 		if (*addr==val) a_spin();
 		else return;
 	}

src/thread/pthread_mutex_timedlock.c

@@ -12,7 +12,7 @@ int pthread_mutex_timedlock(pthread_mutex_t *restrict m, const struct timespec *
 	if (r != EBUSY) return r;
 	
 	int spins = 100;
-	while (spins-- && m->_m_lock) a_spin();
+	while (spins-- && m->_m_lock && !m->_m_waiters) a_spin();
 
 	while ((r=pthread_mutex_trylock(m)) == EBUSY) {
 		if (!(r=m->_m_lock) || ((r&0x40000000) && (m->_m_type&4)))

src/thread/pthread_rwlock_timedrdlock.c

@@ -8,7 +8,7 @@ int pthread_rwlock_timedrdlock(pthread_rwlock_t *restrict rw, const struct times
 	if (r != EBUSY) return r;
 	
 	int spins = 100;
-	while (spins-- && rw->_rw_lock) a_spin();
+	while (spins-- && rw->_rw_lock && !rw->_rw_waiters) a_spin();
 
 	while ((r=pthread_rwlock_tryrdlock(rw))==EBUSY) {
 		if (!(r=rw->_rw_lock) || (r&0x7fffffff)!=0x7fffffff) continue;

src/thread/pthread_rwlock_timedwrlock.c

@@ -8,7 +8,7 @@ int pthread_rwlock_timedwrlock(pthread_rwlock_t *restrict rw, const struct times
 	if (r != EBUSY) return r;
 	
 	int spins = 100;
-	while (spins-- && rw->_rw_lock) a_spin();
+	while (spins-- && rw->_rw_lock && !rw->_rw_waiters) a_spin();
 
 	while ((r=pthread_rwlock_trywrlock(rw))==EBUSY) {
 		if (!(r=rw->_rw_lock)) continue;

src/thread/sem_timedwait.c

@@ -11,7 +11,7 @@ int sem_timedwait(sem_t *restrict sem, const struct timespec *restrict at)
 	if (!sem_trywait(sem)) return 0;
 
 	int spins = 100;
-	while (spins-- && sem->__val[0] <= 0) a_spin();
+	while (spins-- && sem->__val[0] <= 0 && !sem->__val[1]) a_spin();
 
 	while (sem_trywait(sem)) {
 		int r;