Re: [RFC v3 1/1] fs/namespace: remove RCU sync for MNT_DETACH umount

[Christian Brauner <brauner@kernel.org>]

On Fri, Jun 28, 2024 at 01:13:45PM GMT, Jan Kara wrote:
> On Fri 28-06-24 10:58:54, Ian Kent wrote:
> > 
> > On 27/6/24 19:54, Jan Kara wrote:
> > > 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?
> > 
> > Interesting, because of the definition of lazy umount I didn't look closely
> > enough at that.
> > 
> > But I wonder, how exactly would that race occur, is holding the rcu read
> > lock sufficient since the rcu'd mount free won't be done until it's
> > released (at least I think that's how rcu works).
> 
> I'm concerned about a race like:
> 
> [path lookup]				[umount -l]
> ...
> path_put()
>   mntput(mnt)
>     mntput_no_expire(m)
>       rcu_read_lock();
>       if (likely(READ_ONCE(mnt->mnt_ns))) {
> 					do_umount()
> 					  umount_tree()
> 					    ...
> 					    mnt->mnt_ns = NULL;
> 					    ...
> 					  namespace_unlock()
> 					    mntput(&m->mnt)
> 					      mntput_no_expire(mnt)
> 				              smp_mb();
> 					      mnt_add_count(mnt, -1);
> 					      count = mnt_get_count(mnt);
> 					      if (count != 0) {
> 						...
> 						return;
>         mnt_add_count(mnt, -1);
>         rcu_read_unlock();
>         return;
> -> KABOOM, mnt->mnt_count dropped to 0 but nobody cleaned up the mount!
>       }

Yeah, I think that's a valid concern. mntput_no_expire() requires that
the last reference is dropped after an rcu grace period and that can
only be done by synchronize_rcu_*() (It could be reworked but that would
be quite ugly.). See also mnt_make_shortterm() caller's for kernel
initiated unmounts.