static struct dentry *lookup_slow(const struct qstr *name,
				  struct dentry *dir,
				  unsigned int flags)
{
	struct dentry *dentry = ERR_PTR(-ENOENT), *old;
	struct inode *inode = dir->d_inode;
	DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);

	inode_lock_shared(inode);
	/* Don't go there if it's already dead */
	if (unlikely(IS_DEADDIR(inode)))
		goto out;
again:
	dentry = d_alloc_parallel(dir, name, &wq);
	if (IS_ERR(dentry))
		goto out;
	if (unlikely(!d_in_lookup(dentry))) {
		if (!(flags & LOOKUP_NO_REVAL)) {
			int error = d_revalidate(dentry, flags);
			if (unlikely(error <= 0)) {
				if (!error) {
					d_invalidate(dentry);
					dput(dentry);
					goto again;
				}
				dput(dentry);
				dentry = ERR_PTR(error);
			}
		}
	} else {
		old = inode->i_op->lookup(inode, dentry, flags);
		d_lookup_done(dentry);
		if (unlikely(old)) {
			dput(dentry);
			dentry = old;
		}
	}
out:
	inode_unlock_shared(inode);
	return dentry;
}

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.