如何避免互斥变量被同一线程两次锁定？

Question

queueLIFO is QList

//  This function is run by the thread `Producer`.
void *threadProducerFunction (void *arg)
{
    Q_UNUSED (arg);

    while (1)
    {
        if (queueLIFO.length () < 10)
        {
            pthread_mutex_lock (&mutexVariable);
            queueLIFO.push_back (1);
            pthread_mutex_unlock (&mutexVariable);
        }
        else
        {
            pthread_mutex_lock (&mutexVariable);
            pthread_cond_wait (&conditionVariable, &mutexVariable);
        }
    }
    return NULL;
}

Now, considering the following info from this link: https://computing.llnl.gov/tutorials/pthreads/#ConVarSignal

pthread_cond_wait() - this routine should be called while mutex is locked, and it will automatically release the mutex while it waits.

After signal is received and thread is awakened, mutex will be automatically locked for use by the thread.

The programmer is then responsible for unlocking mutex when the thread is finished with it.

When the signal is received from the other thread, pthread_cond_wait will lock the mutex for this thread's usage, which means that in my situation the control will go in the if statement where the mutex is already locked by pthread_cond_wait (from else condition) and we are locking it again now.

Have I written the code logic in a wrong way? How?

Answer 1

You should always hold the lock before you check the condition.

pthread_mutex_lock (&mutexVariable);
while (queueLIFO.length() >= 10) {
    pthread_cond_wait (&conditionVariable, &mutexVariable);
}
queueLIFO.push_back (1);
pthread_mutex_unlock (&mutexVariable);