mtx_lock(unsigned long *lock) {
	unsigned long l;

retry:
	l = *lock;
	if (l == 0) {
		if (cmpxchg(lock, 0, 1)) // cmpxchg returns 1 on failure
					 // cmpxchg(addr, old, new)
			goto retry;
		return;
	}
	
	current->next = l & MASK;
	if (cmpxchg(lock, l, current | 1))
		goto retry;

	sleep_on_queue(current);
}

mtx_unlock(unsigned long *lock) {
	unsigned long l;
	struct thread *head, *p;

retry:
	l = *lock;
	if (l == 1) {
		if (cmpxchg(lock, 1, 0))
			goto retry;
		return;
	}
	head = (struct thread *)(l & MASK);

	// remove current from queue
	list_for_each(p, head) {
		if (p->next == current) {
			p->next = current->next;
			break;
		}
	}

	if (cmpxchg(lock, head | 1, head->next))
		goto retry;

	wake_up_one(head);
}