6sgN7riQ.go

func (l *list) Add(item []byte) bool {
	key, _ := hashstructure.Hash(item, nil)
	l.head.lock()
	pred := l.head

	curr := pred.next
	curr.lock()

	// Displace locks during each iteration.
        // We keep the lock for curr when unlocking pred. 
        // This way the coupled-lock is maintained
	for curr.key < key {
		pred.unlock()
		pred = curr
		curr = curr.next
		curr.lock()
	}

	if curr.key == key {
		pred.unlock()
		curr.unlock()
		return false
	}

	n := newNode(item)
	n.next = curr
	pred.next = n

	l.mu.Lock()
	l.size++
	l.mu.Unlock()

	curr.unlock()
	pred.unlock()
	return true
}