Re: [WTFoTW] ->quota_on() deadlocks

[Jan Kara <jack@suse.cz>]

On Thu 01-07-10 19:56:29, Al Viro wrote:
> 	All quotactl callbacks are done with s_umount held shared.
> Fine, but ->quota_on() will do kern_path() and _that_ can try to
> grab the same thing exclusive - suppose we pass a pathname that
> walks into autofs and triggers mounting of the same fs (at a different
> mountpoint, that is).  That'll end up calling sget(), finding our
> superblock and trying to grab s_umount on it.  mount(8) sits
> uninterruptibly sleeping in mount(2), kern_path() waits for it
> to complete and that's not going to happen until the caller of
> kern_path() (do_quotactl(), ultimately) finishes.
> 
> 	Obvious solution is b0rken - we _can't_ take the call of
> kern_path() to a point prior to getting (and locking) the superblock.
> Why?  Because ocfs2 ignores the pathname argument, so failing on
> bogus pathnames will blow the userland API compatibility.
> 
> 	Other alternatives are also not particulary pleasant since
> we need s_umount at some point there - we want some exclusion with
> remounting.
> 
> 	Ideas?
  Not now :(. I'm on vacation this and next week. I'll look into the bug when
I return.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR