class MCSLock {
  private:
    Queue *tail = NULL;
}

MCSLock::release() {

    if (compare_and_swap(&tail, 
                    myTCB, NULL)) {

        // If tail == myTCB, no one is 
        // waiting. MCSLock is now free.

    } else { 
        // Someone is waiting.
        while (myTCB->next == NULL)
            ; // spin until next is set 

        // Tell next thread to proceed.
        myTCB->next->needToWait = FALSE;
    }
}

MCSLock::acquire() {
    Queue *oldTail = tail;

    myTCB->next = NULL;
    while (!compare_and_swap(&tail,
                  oldTail, &myTCB)) {

        // Try again if someone else
        // changed tail in the meantime.

        oldTail = tail;
    }

    // If oldTail == NULL, lock acquired.
    if (oldTail != NULL) {
        // Need to wait.
        myTCB->needToWait = TRUE;
        memory_barrier();
        oldTail->next = myTCB;
        while (myTCB->needToWait)
            ; //spin
    }
}