static inline struct dentry *lock_parent(struct dentry *dentry)
{
	struct dentry *parent = dentry->d_parent;
	if (IS_ROOT(dentry))
		return NULL;
	if (unlikely(dentry->d_lockref.count < 0))
		return NULL;
	if (likely(spin_trylock(&parent->d_lock)))
		return parent;
	rcu_read_lock();
	spin_unlock(&dentry->d_lock);
again:
	parent = ACCESS_ONCE(dentry->d_parent);
	spin_lock(&parent->d_lock);
	/*
	 * We can't blindly lock dentry until we are sure
	 * that we won't violate the locking order.
	 * Any changes of dentry->d_parent must have
	 * been done with parent->d_lock held, so
	 * spin_lock() above is enough of a barrier
	 * for checking if it's still our child.
	 */
	if (unlikely(parent != dentry->d_parent)) {
		spin_unlock(&parent->d_lock);
		goto again;
	}
	rcu_read_unlock();
	if (parent != dentry)
		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
	else
		parent = NULL;
	return parent;
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.