class FIFOMutex {
   private final AtomicBoolean locked = new AtomicBoolean(false);
   private final Queue<Thread> waiters
     = new ConcurrentLinkedQueue<Thread>();

   public void lock() {
     boolean wasInterrupted = false;
     Thread current = Thread.currentThread();
     waiters.add(current);

     // Block while not first in queue or cannot acquire lock
     while (waiters.peek() != current ||
            !locked.compareAndSet(false, true)) {
        LockSupport.park(this);
        if (Thread.interrupted()) // ignore interrupts while waiting
          wasInterrupted = true;
     }

     waiters.remove();
     if (wasInterrupted)          // reassert interrupt status on exit
        current.interrupt();
   }

   public void unlock() {
     locked.set(false);
     LockSupport.unpark(waiters.peek());
   }
}

class FIFOMutex {
   private final AtomicBoolean locked = new AtomicBoolean(false);
   private final Queue<Thread> waiters
     = new ConcurrentLinkedQueue<Thread>();

   public void lock() {
     boolean wasInterrupted = false;
     Thread current = Thread.currentThread();
     waiters.add(current);

     // Block while not first in queue or cannot acquire lock
     while (waiters.peek() != current ||
            !locked.compareAndSet(false, true)) {
        LockSupport.park(this);
        if (Thread.interrupted()) // ignore interrupts while waiting
          wasInterrupted = true;
     }

     waiters.remove();
     if (wasInterrupted)          // reassert interrupt status on exit
        current.interrupt();
   }

   public void unlock() {
     locked.set(false);
     LockSupport.unpark(waiters.peek());
   }
}