struct dentry *dget_parent(struct dentry *dentry)
{
	int gotref;
	struct dentry *ret;

	/*
	 * Do optimistic parent lookup without any
	 * locking.
	 */
	rcu_read_lock();
	ret = ACCESS_ONCE(dentry->d_parent);
	gotref = lockref_get_not_zero(&ret->d_lockref);
	rcu_read_unlock();
	if (likely(gotref)) {
		if (likely(ret == ACCESS_ONCE(dentry->d_parent)))
			return ret;
		dput(ret);
	}

repeat:
	/*
	 * Don't need rcu_dereference because we re-check it was correct under
	 * the lock.
	 */
	rcu_read_lock();
	ret = dentry->d_parent;
	spin_lock(&ret->d_lock);
	if (unlikely(ret != dentry->d_parent)) {
		spin_unlock(&ret->d_lock);
		rcu_read_unlock();
		goto repeat;
	}
	rcu_read_unlock();
	BUG_ON(!ret->d_lockref.count);
	ret->d_lockref.count++;
	spin_unlock(&ret->d_lock);
	return ret;
}

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.