Why does pthread_cond_wait have spurious wakeups

c++pthreads

To quote the man page:

When using condition variables there is always a Boolean predicate involving shared variables associated with each condition wait that is true if the thread should proceed. Spurious wakeups from the pthread_cond_timedwait() or pthread_cond_wait() functions may occur. Since the return from pthread_cond_timedwait() or pthread_cond_wait() does not imply anything about the value of this predicate, the predicate should be re-evaluated upon such return.

So, pthread_cond_wait can return even if you haven't signaled it. At first glance at least, that seems pretty atrocious. It would be like a function which randomly returned the wrong value or randomly returned before it actually reached a proper return statement. It seems like a major bug. But the fact that they chose to document this in the man page rather than fix it would seem to indicate that there is a legitimate reason why pthread_cond_wait ends up waking up spuriously. Presumably, there's something intrinsic about how it works that makes it so that that can't be helped. The question is what.

Why does pthread_cond_wait return spuriously? Why can't it guarantee that it's only going to wake up when it's been properly signaled? Can anyone explain the reason for its spurious behavior?

Best Solution

There are at least two things 'spurious wakeup' could mean:

  • A thread blocked in pthread_cond_wait can return from the call even though no call to pthread_call_signal or pthread_cond_broadcast on the condition occurred.
  • A thread blocked in pthread_cond_wait returns because of a call to pthread_cond_signal or pthread_cond_broadcast, however after reacquiring the mutex the underlying predicate is found to no longer be true.

But the latter case can occur even if the condition variable implementation does not allow the former case. Consider a producer consumer queue, and three threads.

  • Thread 1 has just dequeued an element and released the mutex, and the queue is now empty. The thread is doing whatever it does with the element it acquired on some CPU.
  • Thread 2 attempts to dequeue an element, but finds the queue to be empty when checked under the mutex, calls pthread_cond_wait, and blocks in the call awaiting signal/broadcast.
  • Thread 3 obtains the mutex, inserts a new element into the queue, notifies the condition variable, and releases the lock.
  • In response to the notification from thread 3, thread 2, which was waiting on the condition, is scheduled to run.
  • However before thread 2 manages to get on the CPU and grab the queue lock, thread 1 completes its current task, and returns to the queue for more work. It obtains the queue lock, checks the predicate, and finds that there is work in the queue. It proceeds to dequeue the item that thread 3 inserted, releases the lock, and does whatever it does with the item that thread 3 enqueued.
  • Thread 2 now gets on a CPU and obtains the lock, but when it checks the predicate, it finds that the queue is empty. Thread 1 'stole' the item, so the wakeup appears to be spurious. Thread 2 needs to wait on the condition again.

So since you already always need to check the predicate under a loop, it makes no difference if the underlying condition variables can have other sorts of spurious wakeups.