When is a condition variable needed, isn't a mutex enough?

Question

I\'m sure mutex isn\'t enough that\'s the reason the concept of condition variables exist; but it beats me and I\'m not able to convince myself with a concrete scenario when

Answer 1

I think it is implementation defined.
The mutex is enough or not depends on whether you regard the mutex as a mechanism for critical sections or something more.

As mentioned in http://en.cppreference.com/w/cpp/thread/mutex/unlock,

The mutex must be locked by the current thread of execution, otherwise, the behavior is undefined.

which means a thread could only unlock a mutex which was locked/owned by itself in C++.
But in other programming languages, you might be able to share a mutex between processes.

So distinguishing the two concepts may be just performance considerations, a complex ownership identification or inter-process sharing are not worthy for simple applications.

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For example, you may fix @slowjelj's case with an additional mutex (it might be an incorrect fix):

Thread1:

lock(mutex0);
while(1) {
    lock(mutex0); // Blocks waiting for notification from Thread2
    ... // do work after notification is received
    unlock(mutex1); // Tells Thread2 we are done
}

Thread2:

while(1) {
    lock(mutex1); // lock the mutex so Thread1 will block again
    ... // do the work that precedes notification
    unlock(mutex0); // unblocks Thread1
}

But your program will complain that you have triggered an assertion left by the compiler (e.g. "unlock of unowned mutex" in Visual Studio 2015).