Proper use of condition variables?

[David Xu]

Ivan Voras wrote:
> 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);
> 

	use :
		while (STAILQ_EMPTY(&thr->queue))
			pthread_cond_wait(&thr->queue_cv, &thr->queue_mtx);

It is possible pthread_cond_wait returned with surplus wakeup, e.g
kernel signal etcs...


> 	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].