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

[Al Viro <viro@ZenIV.linux.org.uk>]

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?

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