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authorRich Felker <dalias@aerifal.cx>2015-02-22 22:07:50 -0500
committerRich Felker <dalias@aerifal.cx>2015-02-22 22:07:50 -0500
commit8741ffe625363a553e8f509dc3ca7b071bdbab47 (patch)
treef917a4e8ce76450e2e956b051912cb9861fd9701 /src/thread
parent102f6a01e249ce4495f1119ae6d963a2a4a53ce5 (diff)
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fix pthread_cond_wait cancellation race
it's possible that signaling a waiter races with cancellation of that same waiter. previously, cancellation was acted upon, causing the signal to be consumed with no waiter returning. by using the new masked cancellation state, it's possible to refuse to act on the cancellation request and instead leave it pending. to ease review and understanding of the changes made, this commit leaves the unwait function, which was previously the cancellation cleanup handler, in place. additional simplifications could be made by removing it.
Diffstat (limited to 'src/thread')
-rw-r--r--src/thread/pthread_cond_timedwait.c43
1 files changed, 38 insertions, 5 deletions
diff --git a/src/thread/pthread_cond_timedwait.c b/src/thread/pthread_cond_timedwait.c
index 0a80d305..5f29a249 100644
--- a/src/thread/pthread_cond_timedwait.c
+++ b/src/thread/pthread_cond_timedwait.c
@@ -32,7 +32,7 @@ struct waiter {
int *notify;
pthread_mutex_t *mutex;
pthread_cond_t *cond;
- int shared;
+ int shared, err;
};
/* Self-synchronized-destruction-safe lock functions */
@@ -73,6 +73,11 @@ static void unwait(void *arg)
if (node->shared) {
pthread_cond_t *c = node->cond;
pthread_mutex_t *m = node->mutex;
+ /* Suppress cancellation if a signal was potentially
+ * consumed; this is a legitimate form of spurious
+ * wake even if not. */
+ if (node->err == ECANCELED && c->_c_seq != node->state)
+ node->err = 0;
if (a_fetch_add(&c->_c_waiters, -1) == -0x7fffffff)
__wake(&c->_c_waiters, 1, 0);
node->mutex_ret = pthread_mutex_lock(m);
@@ -121,12 +126,23 @@ static void unwait(void *arg)
} else {
a_dec(&node->mutex->_m_waiters);
}
+
+ /* Since a signal was consumed, acting on cancellation is not
+ * permitted. The only other error possible at this stage,
+ * ETIMEDOUT, is permitted even if a signal was consumed. */
+ if (node->err = ECANCELED) node->err = 0;
}
+static void dummy(void *arg)
+{
+}
+
+int __pthread_setcancelstate(int, int *);
+
int __pthread_cond_timedwait(pthread_cond_t *restrict c, pthread_mutex_t *restrict m, const struct timespec *restrict ts)
{
struct waiter node = { .cond = c, .mutex = m };
- int e, seq, *fut, clock = c->_c_clock;
+ int e, seq, *fut, clock = c->_c_clock, cs;
if ((m->_m_type&15) && (m->_m_lock&INT_MAX) != __pthread_self()->tid)
return EPERM;
@@ -139,7 +155,7 @@ int __pthread_cond_timedwait(pthread_cond_t *restrict c, pthread_mutex_t *restri
if (c->_c_shared) {
node.shared = 1;
fut = &c->_c_seq;
- seq = c->_c_seq;
+ seq = node.state = c->_c_seq;
a_inc(&c->_c_waiters);
} else {
lock(&c->_c_lock);
@@ -157,13 +173,30 @@ int __pthread_cond_timedwait(pthread_cond_t *restrict c, pthread_mutex_t *restri
__pthread_mutex_unlock(m);
- do e = __timedwait(fut, seq, clock, ts, unwait, &node, !node.shared);
+ __pthread_setcancelstate(PTHREAD_CANCEL_MASKED, &cs);
+
+ do e = __timedwait(fut, seq, clock, ts, dummy, 0, !node.shared);
while (*fut==seq && (!e || e==EINTR));
if (e == EINTR) e = 0;
+ node.err = e;
unwait(&node);
+ e = node.err;
+
+ /* Suppress cancellation if there was an error locking the mutex,
+ * since the contract for cancellation requires the mutex to be
+ * locked when the cleanup handler is called, and there is no
+ * way to report an error. */
+ if (node.mutex_ret) e = node.mutex_ret;
+
+ __pthread_setcancelstate(cs, 0);
+
+ if (e == ECANCELED) {
+ __pthread_testcancel();
+ __pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, 0);
+ }
- return node.mutex_ret ? node.mutex_ret : e;
+ return e;
}
int __private_cond_signal(pthread_cond_t *c, int n)