Condition variable deadlock

Question

I have a problem with a deadlock in my code related to the use of condition variables. This is more of a design question than a pure code question. I have no problem actually writing code once I understand the correct design. I have the following scenario:

1. Thread A waits on a condition variable.
2. Thread B calls notify_all, and thread A wakes up.

This is of course what I want to happen, and is what does happen when everything works as expected. But sometimes, I get the following scenario instead:

1. Thread A executes the code right before it begins to wait on the condition variable.
2. Thread B calls notify_all, thinking that thread A is waiting.
3. Thread A begins waiting on the condition variable, not realizing that thread B already told it to stop waiting. Deadlock.

What is the best way to resolve this? I can't think of a reliable way to check whether thread A is actually waiting, in order to know when I should call notify_all in thread B. Do I have to resort to timed_lock? I would hate to.

Answer 1

During the period just before Thread A waits on condition variable it must be holding a mutex. The easiest solution is to make sure that Thread B is holding the same mutex at the time it calls notify_all. So something like this:

std::mutex m;
std::condition_variable cv;
int the_condition = 0;

Thread A: {
  std::unique_lock<std::mutex> lock(m);
  do something
  while (the_condition == 0) {
    cv.wait(lock);
  }
  now the_condition != 0 and thread A has the mutex
  do something else
} // releases the mutex;

Thread B: {
  std::unique_lock<std::mutex> lock(m);
  do something that makes the_condition != 0
  cv.notify_all();
} // releases the mutex

This guarantees that Thread B only does the notify_all() either before Thread A acquires the mutex or while Thread A is waiting on the condition variable.

The other key here, though, is the while loop waiting for the_condition to become true. Once A has the mutex it should not be possible for any other thread to change the_condition until A has tested the_condition, found it false, and started waiting (thus releasing the mutex).

The point is: what you are really waiting for is for the value of the_condition to become non-zero, the std::condition_variable::notify_all is just telling you that thread B thinks thread A should wake up and retest.

Answer 2

A condition variable must always be associated with a mutex to avoid a race condition created by one thread preparing to wait and another thread which may signal the condition before the first thread actually waits on it resulting in a deadlock. The thread will be perpetually waiting for a signal that is never sent. Any mutex can be used, there is no explicit link between the mutex and the condition variable.