linux-kernel - Re: [RFC v3 1/1] fs/namespace: remove RCU sync for MNT

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Message-ID: <20240627115418.lcnpctgailhlaffc@quack3>
Date: Thu, 27 Jun 2024 13:54:18 +0200
From: Jan Kara <jack@...e.cz>
To: Ian Kent <ikent@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>,
	Lucas Karpinski <lkarpins@...hat.com>, viro@...iv.linux.org.uk,
	brauner@...nel.org, jack@...e.cz, raven@...maw.net,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alexander Larsson <alexl@...hat.com>,
	Eric Chanudet <echanude@...hat.com>
Subject: Re: [RFC v3 1/1] fs/namespace: remove RCU sync for MNT_DETACH umount

On Thu 27-06-24 09:11:14, Ian Kent wrote:
> On 27/6/24 04:47, Matthew Wilcox wrote:
> > On Wed, Jun 26, 2024 at 04:07:49PM -0400, Lucas Karpinski wrote:
> > > +++ b/fs/namespace.c
> > > @@ -78,6 +78,7 @@ static struct kmem_cache *mnt_cache __ro_after_init;
> > >   static DECLARE_RWSEM(namespace_sem);
> > >   static HLIST_HEAD(unmounted);	/* protected by namespace_sem */
> > >   static LIST_HEAD(ex_mountpoints); /* protected by namespace_sem */
> > > +static bool lazy_unlock = false; /* protected by namespace_sem */
> > That's a pretty ugly way of doing it.  How about this?
> 
> Ha!
> 
> That was my original thought but I also didn't much like changing all the
> callers.
> 
> I don't really like the proliferation of these small helper functions either
> but if everyone
> 
> is happy to do this I think it's a great idea.

So I know you've suggested removing synchronize_rcu_expedited() call in
your comment to v2. But I wonder why is it safe? I *thought*
synchronize_rcu_expedited() is there to synchronize the dropping of the
last mnt reference (and maybe something else) - see the comment at the
beginning of mntput_no_expire() - and this change would break that?

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR