Date: Sun, 23 Dec 2007 00:10:53 +0100 From: Ivan Voras <ivoras@freebsd.org> To: freebsd-threads@freebsd.org Subject: Proper use of condition variables? Message-ID: <fkk5ht$gsj$1@ger.gmane.org>
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Hi,
I'm implementing what is basically a producer-consumer setup in which
two threads communicate only by a message queue. The idea is that the
consumer waits on a condition variable until something gets in the
queue, then takes it out and processes it. Unfortunately the program
deadlocks with the provider waiting in pthread_mutex_lock(queue_mtx) and
the consumer waiting in pthread_cond_wait(queue_cv, queue_mtx). This is
on RELENG_7. Am I misreading how pthread_cond_wait should behave? I
thought it should release the mutex until the cv gets signaled.
On the consumer side, the code looks like this:
while (1) {
pthread_mutex_lock(&thr->queue_mtx);
if (STAILQ_EMPTY(&thr->queue))
[X] pthread_cond_wait(&thr->queue_cv, &thr->queue_mtx);
job = STAILQ_FIRST(&thr->queue);
STAILQ_REMOVE_HEAD(&thr->queue, linkage);
pthread_mutex_unlock(&thr->queue_mtx);
process(job);
}
On the server side, it's like this:
[X] pthread_mutex_lock(&thr->queue_mtx);
STAILQ_INSERT_TAIL(&thr->queue, job, linkage);
pthread_mutex_unlock(&thr->queue_mtx);
pthread_cond_signal(&thr->queue_cv);
The two lines that deadlock are marked with [X].
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