From: Rich Felker <dalias@aerifal.cx>
Date: Tue, 26 Aug 2014 00:24:07 +0000 (-0400)
Subject: refrain from spinning on locks when there is already a waiter
X-Git-Tag: v1.1.5~48
X-Git-Url: https://git.librecmc.org/?a=commitdiff_plain;h=f5fb20b0e934770c37093105524ea644dcaba5e2;p=oweals%2Fmusl.git

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.
---

diff --git a/src/thread/__wait.c b/src/thread/__wait.c
index c1d6b61c..01ee5982 100644
--- a/src/thread/__wait.c
+++ b/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;
 	}
diff --git a/src/thread/pthread_mutex_timedlock.c b/src/thread/pthread_mutex_timedlock.c
index 116a8b7b..ae883f90 100644
--- a/src/thread/pthread_mutex_timedlock.c
+++ b/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)))
diff --git a/src/thread/pthread_rwlock_timedrdlock.c b/src/thread/pthread_rwlock_timedrdlock.c
index 884b7a1e..ea50da4a 100644
--- a/src/thread/pthread_rwlock_timedrdlock.c
+++ b/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;
diff --git a/src/thread/pthread_rwlock_timedwrlock.c b/src/thread/pthread_rwlock_timedwrlock.c
index f02b174b..8d04f561 100644
--- a/src/thread/pthread_rwlock_timedwrlock.c
+++ b/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;
diff --git a/src/thread/sem_timedwait.c b/src/thread/sem_timedwait.c
index df5f3a6c..b5a60add 100644
--- a/src/thread/sem_timedwait.c
+++ b/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;