Question

If I lock the same mutex in two different places in my function, and a context switch occurs when one thread is in one of them, and the second thread gets to the other one, will it be blocked?

I’ll try to give a simple example of what I mean, maybe it will be clearer.
Say I have the following code in a file test.c

int globalVar = 0;    

void testMutex(pthread_mutex_t myMutex) {
    pthread_mutex_lock(&myMutex);
    globalVar++;
    pthread_mutex_unlock(&myMutex);

    printf("%s \n", "Doing some other stuff here");

    pthread_mutex_lock(&myMutex);
    globalVar--;
    pthread_mutex_unlock(&myMutex);
}

and in a different file main.c, I have a main function which create two threads, thread1 and thread2, both running testMutex function.
thread1 is executed first, and while in the second part of the function (the — part, after printf), before mutex is unlocked, thread2 start running, from the function beginning.
will thread2 be able to execute globalVar++, or will it stay blocked, waiting for the mutex to be unlocked?

Thanks in advance!

Answer 1

The short answer is “yes”.

The pthread_mutex_lock documentation makes this pretty clear:

The mutex object referenced by mutex shall be locked by calling pthread_mutex_lock(). If the mutex is already locked, the calling thread shall block until the mutex becomes available. This operation shall return with the mutex object referenced by mutex in the locked state with the calling thread as its owner.

In other words, upon return from pthread_mutex_lock, the mutex is “owned” by the thread. The system guarantees that at most one thread can “own” a single mutex at any time.