Q. hlist_bl_add_head_rcu() in d_alloc_parallel()

From: "J. R. Okajima" <hooanon05g@gmail.com>
To: viro@ZenIV.linux.org.uk, linux-fsdevel@vger.kernel.org
Subject: Q. hlist_bl_add_head_rcu() in d_alloc_parallel()
Date: Sat, 18 Jun 2016 05:50:30 +0900	[thread overview]
Message-ID: <13136.1466196630@jrobl> (raw)

I am afraid there may exist another violation of "no lookups on the same
name in parallel" rule, but I am not sure.

Roughly d_alloc_parallel() behaves like this.

struct dentry *d_alloc_parallel()
{
	new = d_alloc(parent, name);

	rcu_read_lock();
	hlist_bl_lock(b);
	rcu_read_unlock();
	hlist_bl_for_each_entry(dentry, node, b, d_u.d_in_lookup_hash) {
		if (!matched_dentry_found)
			continue;
		dget(dentry);
		hlist_bl_unlock(b);
		return dentry;
	}
	hlist_bl_add_head_rcu(&new->d_u.d_in_lookup_hash, b);
	hlist_bl_unlock(b);
	return new;
}

When two processes try opening a single existing file and enters
d_alloc_parallel() at the same time, only one process wins and should
succeeds hlist_bl_add_head_rcu(). The other process should find the
dentry in d_u.d_in_lookup_hash and return 'dentry' (instead of
'new'). Am I right?

My question is when will 'new' be added into d_u.d_in_lookup_hash?
It should be between these two lines, I guess.
	rcu_read_unlock();
	hlist_bl_for_each_entry(dentry, node, b, d_u.d_in_lookup_hash) {
But can it surely happen?
If 'new' is not added here because someone else is in rcu_read_lock
region or other reason, then both processes will add the same named but
different dentry?

Is it better to change the lock/unlock-order like this?

	rcu_read_unlock();
	rcu_barrier();
	hlist_bl_lock(b);
	hlist_bl_for_each_entry(dentry, node, b, d_u.d_in_lookup_hash) {

J. R. Okajima