struct dentry *d_alloc_parallel(struct dentry *parent,
				const struct qstr *name,
				wait_queue_head_t *wq)
{
	unsigned int hash = name->hash;
	struct hlist_bl_head *b = in_lookup_hash(parent, hash);
	struct hlist_bl_node *node;
	struct dentry *new = d_alloc(parent, name);
	struct dentry *dentry;
	unsigned seq, r_seq, d_seq;

	if (unlikely(!new))
		return ERR_PTR(-ENOMEM);

retry:
	rcu_read_lock();
	seq = smp_load_acquire(&parent->d_inode->i_dir_seq) & ~1;
	r_seq = read_seqbegin(&rename_lock);
	dentry = __d_lookup_rcu(parent, name, &d_seq);
	if (unlikely(dentry)) {
		if (!lockref_get_not_dead(&dentry->d_lockref)) {
			rcu_read_unlock();
			goto retry;
		}
		if (read_seqcount_retry(&dentry->d_seq, d_seq)) {
			rcu_read_unlock();
			dput(dentry);
			goto retry;
		}
		rcu_read_unlock();
		dput(new);
		return dentry;
	}
	if (unlikely(read_seqretry(&rename_lock, r_seq))) {
		rcu_read_unlock();
		goto retry;
	}
	hlist_bl_lock(b);
	if (unlikely(parent->d_inode->i_dir_seq != seq)) {
		hlist_bl_unlock(b);
		rcu_read_unlock();
		goto retry;
	}
	/*
	 * No changes for the parent since the beginning of d_lookup().
	 * Since all removals from the chain happen with hlist_bl_lock(),
	 * any potential in-lookup matches are going to stay here until
	 * we unlock the chain.  All fields are stable in everything
	 * we encounter.
	 */
	hlist_bl_for_each_entry(dentry, node, b, d_u.d_in_lookup_hash) {
		if (dentry->d_name.hash != hash)
			continue;
		if (dentry->d_parent != parent)
			continue;
		if (!d_same_name(dentry, parent, name))
			continue;
		hlist_bl_unlock(b);
		/* now we can try to grab a reference */
		if (!lockref_get_not_dead(&dentry->d_lockref)) {
			rcu_read_unlock();
			goto retry;
		}

		rcu_read_unlock();
		/*
		 * somebody is likely to be still doing lookup for it;
		 * wait for them to finish
		 */
		spin_lock(&dentry->d_lock);
		d_wait_lookup(dentry);
		/*
		 * it's not in-lookup anymore; in principle we should repeat
		 * everything from dcache lookup, but it's likely to be what
		 * d_lookup() would've found anyway.  If it is, just return it;
		 * otherwise we really have to repeat the whole thing.
		 */
		if (unlikely(dentry->d_name.hash != hash))
			goto mismatch;
		if (unlikely(dentry->d_parent != parent))
			goto mismatch;
		if (unlikely(d_unhashed(dentry)))
			goto mismatch;
		if (unlikely(!d_same_name(dentry, parent, name)))
			goto mismatch;
		/* OK, it *is* a hashed match; return it */
		spin_unlock(&dentry->d_lock);
		dput(new);
		return dentry;
	}
	rcu_read_unlock();
	/* we can't take ->d_lock here; it's OK, though. */
	new->d_flags |= DCACHE_PAR_LOOKUP;
	new->d_wait = wq;
	hlist_bl_add_head_rcu(&new->d_u.d_in_lookup_hash, b);
	hlist_bl_unlock(b);
	return new;
mismatch:
	spin_unlock(&dentry->d_lock);
	dput(dentry);
	goto retry;
}

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.