Re: [GIT PULL] Ceph fixes for 5.1-rc7

[Al Viro]

On Fri, Apr 26, 2019 at 09:36:00AM -0700, Linus Torvalds wrote:
> On Fri, Apr 26, 2019 at 9:25 AM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> >
> > Is it really ok to union the count and rcu_head there?
> 
> It should be fine, because the rcu_count should only ever be used once
> the count has gone to zero and the name cannot be found any more.
> 
> And while RCU path walking may find and use the *name* after the
> dentry has been killed off (but not free'd yet), all the actual
> external_name() accesses should be serialized by the dentry lock, so
> there's no access to those fields once the dentry is dead.

It's not quite that; access to external_name contents is fine,
->d_lock or not.  __d_lookup_rcu() does read it under rcu_read_lock
alone.

However:
	* we never free it without an RCU delay after the final
drop of refcount.  RCU delay might happen on dentry->d_rcu (if
it's dentry_free()) or on name->p.rcu (if it's release_dentry_name_snapshot()
or d_move() dropping the final reference).
	* it's never observed in ->d_name after the refcount
reaches zero.
	* no lockless access ever looks at the refcount.  It
can look at ->name[], but that's it.

What I don't understand is why would anyone want to mess with
name snapshots for dentry_path() lookalikes...