Editing Monitor (synchronization) (section)

====Spin-waiting====
One naive approach to achieve synchronization, as alluded to above, is to use "'''spin-waiting'''", in which a mutex is used to protect the critical sections of code and busy-waiting is still used, with the lock being acquired and released in between each busy-wait check.

<syntaxhighlight lang="cpp">
global RingBuffer queue; // A thread-unsafe ring-buffer of tasks.
global Lock queueLock; // A mutex for the ring-buffer of tasks.

// Method representing each producer thread's behavior:
public method producer() {
    while (true) {
        task myTask = ...; // Producer makes some new task to be added.

        queueLock.acquire(); // Acquire lock for initial busy-wait check.
        while (queue.isFull()) { // Busy-wait until the queue is non-full.
            queueLock.release();
            // Drop the lock temporarily to allow a chance for other threads
            // needing queueLock to run so that a consumer might take a task.
            queueLock.acquire(); // Re-acquire the lock for the next call to "queue.isFull()".
        }

        queue.enqueue(myTask); // Add the task to the queue.
        queueLock.release(); // Drop the queue lock until we need it again to add the next task.
    }
}

// Method representing each consumer thread's behavior:
public method consumer() {
    while (true) {
        queueLock.acquire(); // Acquire lock for initial busy-wait check.
        while (queue.isEmpty()) { // Busy-wait until the queue is non-empty.
            queueLock.release();
            // Drop the lock temporarily to allow a chance for other threads
            // needing queueLock to run so that a producer might add a task.
            queueLock.acquire(); // Re-acquire the lock for the next call to "queue.isEmpty()".
        }
        myTask = queue.dequeue(); // Take a task off of the queue.
        queueLock.release(); // Drop the queue lock until we need it again to take off the next task.
        doStuff(myTask); // Go off and do something with the task.
    }
}
</syntaxhighlight>

This method assures that an inconsistent state does not occur, but wastes CPU resources due to the unnecessary busy-waiting.  Even if the queue is empty and producer threads have nothing to add for a long time, consumer threads are always busy-waiting unnecessarily.  Likewise, even if consumers are blocked for a long time on processing their current tasks and the queue is full, producers are always busy-waiting.  This is a wasteful mechanism.  What is needed is a way to make producer threads block until the queue is non-full, and a way to make consumer threads block until the queue is non-empty.

(N.B.: Mutexes themselves can also be '''spin-locks''' which involve busy-waiting in order to get the lock, but in order to solve this problem of wasted CPU resources, we assume that ''queueLock'' is not a spin-lock and properly uses a blocking lock queue itself.)