Prioritization of one thread to process a variable when it is shared by 2 threads and protected by pthread_mutex_lock

Question

Could anyone please suggest how to optimize the application code below using pthread_mutex_lock?

Let me describe the situation:
I have 2 threads sharing a global shared memory variable. The variable shmPtr->status is protected with mutex lock in both functions. Although there is a sleep(1/2) inside the "for loop" in the task1 function, I can't access the shmPtr->status in task2 when required and have to wait until the "for loop" is finished in the task1 function. It takes around 50 seconds for the shmPtr->status to be available for the task2 function.

I am wondering why the task1 function is not releasing the mutex lock despite the sleep(1/2) line. I don't want to wait with processing the shmPtr->status in the task2 function. Please advice.

 thr_id1 = pthread_create ( &p_thread1, NULL, (void *)execution_task1, NULL );
 thr_id2 = pthread_create ( &p_thread2, NULL, (void *)execution_task2, NULL );  

 void execution_task1()
 {
     for(int i = 0;i < 100;i++) 
     {
          //100 lines of application code running here
          pthread_mutex_lock(&lock);  
                shmPtr->status = 1;  //shared memory variable 
         pthread_mutex_unlock(&lock);
         sleep(1/2);
     }
 }

 void execution_task2()
 {
         //100 lines of application code running here
         pthread_mutex_lock(&lock);  
                shmPtr->status = 0;  //shared memory variable
         pthread_mutex_unlock(&lock);
         sleep(1/2);
 }

Regards, NK

John Bollinger · Answer 1 · 2020-09-03T17:29:22.413

I am wondering why the task1 function is not releasing the mutex lock even with an sleep(1/2).

There is no reason to think that the thread running execution_task1() in your example fails to release the mutex, though you would know for sure if you appropriately tested the return value of its pthread_mutex_unlock() call. Rather, the potential problem is that it reacquires the mutex before any other thread contending for it has an opportunity to acquire it.

It seems plausible that a call to sleep(1/2) is ineffective at preventing that. 1/2 is an integer division, evaluating to 0, so you are performing a sleep(0). That probably does not cause the calling thread to suspend at all, and it may not even cause the thread to yield the CPU.

More generally, sleep() is never a good solution for any thread-synchronization problem.

If you are running on a multi-core system, however, and maybe even if you aren't, then freezing out other threads by such a mechanism seems unlikely if the function really does execute a hundred lines of code between releasing the mutex and trying to lock it again. If that's what you think you see then look harder.

If you really do need to force a thread to allow others a chance to acquire the mutex, then you could perhaps set up a fair queueing system as described in Implementing a FIFO mutex in pthreads. For a case such as you dsecribe, however, with one long-running thread needing occasionally to yield to other, quicker tasks, you could consider introducing a condition variable on which that long-running thread can suspend, and an atomic counter by which it can determine whether it should do so:

#include <stdatomic.h>

// ...

pthread_cond_t yield_cv = PTHREAD_COND_INITIALIZER;
_Atomic unsigned int waiting_thread_count = ATOMIC_VAR_INIT(0);

void execution_task1() {
    for (int i = 0; i < 100; i++) {
        // ...

        pthread_mutex_lock(&lock);
        if (waiting_thread_count > 0) {
            pthread_cond_wait(&yield_cv, &lock);
            // spurrious wakeup ok
        }

        // ... critical section ...

        pthread_mutex_unlock(&lock);
    }
}

void execution_task2() {
    // ...

    waiting_thread_count += 1;  // Atomic get & increment; safe w/o mutex
    pthread_mutex_lock(&lock);  
    waiting_thread_count -= 1;
    pthread_cond_signal(&yield_cv);  // no problem if no-one is presently waiting

    // ... critical section ...

    pthread_mutex_unlock(&lock);
}

Using an atomic counter relieves the program from having to protect that counter with its own mutex, which could just shift the problem instead of solving it. That allows threads to use the counter to signal upcoming attempts to acquire the mutex. This intent is then visible to the other thread, so that it can suspend on the CV to allow the other to acquire the mutex.

The short-running threads then acknowledge acquiring the mutex by decrementing the counter. They must do so before releasing the mutex, else the long-running thread might cycle around, acquire the mutex, and read the counter before it is decremented, thus erroneously blocking on the CV when no additional signal can be expected.

Although CV's can be subject to spurrious wakeup, that does not present a serious problem for this approach. If the long-running thread wakes spurriously from it wait on the CV, then the worst that happens is that it performs one more iteration of its main loop, then waits again.

Prioritization of one thread to process a variable when it is shared by 2 threads and protected by pthread_mutex_lock

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