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-rw-r--r--src/thread/pthread_cond_timedwait.c62
1 files changed, 22 insertions, 40 deletions
diff --git a/src/thread/pthread_cond_timedwait.c b/src/thread/pthread_cond_timedwait.c
index 7aaba954..52e306b2 100644
--- a/src/thread/pthread_cond_timedwait.c
+++ b/src/thread/pthread_cond_timedwait.c
@@ -10,12 +10,10 @@
* degenerate list of one member.
*
* Waiter lists attached to the condition variable itself are
- * protected by the lock on the cv. Detached waiter lists are
- * protected by the associated mutex. The hand-off between protections
- * is handled by a "barrier" lock in each node, which disallows
- * signaled waiters from making forward progress to the code that will
- * access the list using the mutex until the list is in a consistent
- * state and the cv lock as been released.
+ * protected by the lock on the cv. Detached waiter lists are never
+ * modified again, but can only be traversed in reverse order, and are
+ * protected by the "barrier" locks in each node, which are unlocked
+ * in turn to control wake order.
*
* Since process-shared cond var semantics do not necessarily allow
* one thread to see another's automatic storage (they may be in
@@ -58,7 +56,7 @@ enum {
static void unwait(void *arg)
{
- struct waiter *node = arg, *p;
+ struct waiter *node = arg;
if (node->shared) {
pthread_cond_t *c = node->cond;
@@ -91,49 +89,34 @@ static void unwait(void *arg)
if (a_fetch_add(node->notify, -1)==1)
__wake(node->notify, 1, 1);
}
+ } else {
+ /* Lock barrier first to control wake order. */
+ lock(&node->barrier);
}
node->mutex_ret = pthread_mutex_lock(node->mutex);
if (oldstate == WAITING) return;
- /* If the mutex can't be locked, we're in big trouble because
- * it's all that protects access to the shared list state.
- * In order to prevent catastrophic stack corruption from
- * unsynchronized access, simply deadlock. */
- if (node->mutex_ret && node->mutex_ret != EOWNERDEAD)
- for (;;) lock(&(int){0});
-
- /* Wait until control of the list has been handed over from
- * the cv lock (signaling thread) to the mutex (waiters). */
- lock(&node->barrier);
-
/* If this thread was requeued to the mutex, undo the extra
* waiter count that was added to the mutex. */
if (node->requeued) a_dec(&node->mutex->_m_waiters);
- /* Find a thread to requeue to the mutex, starting from the
- * end of the list (oldest waiters). */
- for (p=node; p->next; p=p->next);
- if (p==node) p=node->prev;
- for (; p && p->requeued; p=p->prev);
- if (p==node) p=node->prev;
- if (p) {
- p->requeued = 1;
+ /* Unlock the barrier that's holding back the next waiter,
+ * and either wake it or requeue it to the mutex. */
+ if (node->prev) {
+ unlock(&node->prev->barrier);
+ node->prev->requeued = 1;
a_inc(&node->mutex->_m_waiters);
/* The futex requeue command cannot requeue from
* private to shared, so for process-shared mutexes,
* simply wake the target. */
int wake = node->mutex->_m_type & 128;
- __syscall(SYS_futex, &p->state, FUTEX_REQUEUE|128,
+ __syscall(SYS_futex, &node->prev->state, FUTEX_REQUEUE|128,
wake, 1, &node->mutex->_m_lock) != -EINVAL
- || __syscall(SYS_futex, &p->state, FUTEX_REQUEUE,
+ || __syscall(SYS_futex, &node->prev->state, FUTEX_REQUEUE,
0, 1, &node->mutex->_m_lock);
}
-
- /* Remove this thread from the list. */
- if (node->next) node->next->prev = node->prev;
- if (node->prev) node->prev->next = node->next;
}
int pthread_cond_timedwait(pthread_cond_t *restrict c, pthread_mutex_t *restrict m, const struct timespec *restrict ts)
@@ -181,7 +164,7 @@ int pthread_cond_timedwait(pthread_cond_t *restrict c, pthread_mutex_t *restrict
int __private_cond_signal(pthread_cond_t *c, int n)
{
- struct waiter *p, *q=0;
+ struct waiter *p, *first=0;
int ref = 0, cur;
lock(&c->_c_lock);
@@ -196,7 +179,7 @@ int __private_cond_signal(pthread_cond_t *c, int n)
p->notify = &ref;
} else {
n--;
- if (!q) q=p;
+ if (!first) first=p;
}
}
/* Split the list, leaving any remainder on the cv. */
@@ -214,12 +197,11 @@ int __private_cond_signal(pthread_cond_t *c, int n)
* signaled threads to proceed. */
while ((cur = ref)) __wait(&ref, 0, cur, 1);
- /* Wake the first signaled thread and unlock the per-waiter
- * barriers preventing their forward progress. */
- for (p=q; p; p=q) {
- q = p->prev;
- if (!p->next) __wake(&p->state, 1, 1);
- unlock(&p->barrier);
+ /* Allow first signaled waiter, if any, to proceed. */
+ if (first) {
+ __wake(&first->state, 1, 1);
+ unlock(&first->barrier);
}
+
return 0;
}