bool is_subdir(struct dentry *new_dentry, struct dentry *old_dentry)
{
	bool result;
	unsigned seq;

	if (new_dentry == old_dentry)
		return true;

	do {
		/* for restarting inner loop in case of seq retry */
		seq = read_seqbegin(&rename_lock);
		/*
		 * Need rcu_readlock to protect against the d_parent trashing
		 * due to d_move
		 */
		rcu_read_lock();
		if (d_ancestor(old_dentry, new_dentry))
			result = true;
		else
			result = false;
		rcu_read_unlock();
	} while (read_seqretry(&rename_lock, seq));

	return result;
}

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.