Proper use of condition variables?

Daniel Eischen deischen at
Sat Dec 22 21:47:17 PST 2007

On Sun, 23 Dec 2007, 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.

That is the way things are suppose to work.  I would try libkse
to see if that makes any difference.

> 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].

I would put some assert()s in for the ptread_mutex_* and
pthread_cond_* calls, to make sure no errors are being returned.

Also, because pthread_cond_signal() is called outside the lock,
it is possible for job to be NULL.  This is because the server
thread may get preempted after pthread_mutex_unlock() and before
pthread_cond_signal() has completed.  In that time, a consumer
thread can run and pull the job out of the queue, process it and
go back to waiting on the CV again.  It is required that
pthread_cond_wait() always be called with a valid mutex and
with that mutex locked.  It is not required for pthread_cond_signal()
to have the mutex locked, but it is generally a good idea to
call pthread_cond_signal() inside the locked region to avoid
any possible problems like this.


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