Re: [PATCH 3/5 v2] superblock: introduce per-sb cache shrinker infrastructure

Al Viro
2010-05-27T03:59:14Z

On Thu, May 27, 2010 at 11:53:35AM +1000, Dave Chinner wrote:
> On Thu, May 27, 2010 at 09:12:14AM +1000, Dave Chinner wrote:
> > On Thu, May 27, 2010 at 02:41:16AM +1000, Nick Piggin wrote:
> ....
> > > Nitpick but I prefer just the restart label wher it is previously. This
> > > is moving setup for the next iteration into the "error" case.
> >
> > Ok, will fix.
> ....
> > > Would you just elaborate on the lock order problem somewhere? (the
> > > comment makes it look like we *could* take the mutex if we wanted
> > > to).
> >
> > The shrinker is unregistered in deactivate_locked_super() which is
> > just before ->kill_sb is called. The sb->s_umount lock is held at
> > this point. hence is the shrinker is operating, we will deadlock if
> > we try to lock it like this:
> >
> > unmount: shrinker:
> > down_read(&shrinker_lock);
> > down_write(&sb->s_umount)
> > unregister_shrinker()
> > down_write(&shrinker_lock)
> > prune_super()
> > down_read(&sb->s_umount);
> > (deadlock)
> >
> > hence if we can't get the sb->s_umount lock in prune_super(), then
> > the superblock must be being unmounted and the shrinker should abort
> > as the ->kill_sb method will clean up everything after the shrinker
> > is unregistered. Hence the down_read_trylock().

Um... Maybe I'm dumb, but what's wrong with doing unregistration from
deactivate_locked_super(), right after the call of ->kill_sb()? At that
point ->s_umount is already dropped, so we won't deadlock at all.
Shrinker rwsem will make sure that all shrinkers-in-progress will run
to completion, so we won't get a superblock freed under prune_super().
I don't particulary mind down_try_read() in prune_super(), but why not
make life obviously safer?

Am I missing something here?