linux-kernel - Re: [BUG] deadlock between configfs_rmdir() and sys_rename() (WAS Re: [RFC][PATCH 4/4] configfs: Make multiple default

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Date:	Tue, 10 Jun 2008 12:14:27 +0200
From:	Louis Rilling <Louis.Rilling@...labs.com>
To:	Joel.Becker@...cle.com
Cc:	ocfs2-devel@....oracle.com, linux-kernel@...r.kernel.org
Subject: Re: [BUG] deadlock between configfs_rmdir() and sys_rename() (WAS
	Re: [RFC][PATCH 4/4] configfs: Make multiple default_group)
	destructions lockdep friendly

On Mon, Jun 09, 2008 at 06:58:00PM -0700, Joel Becker wrote:
> On Mon, Jun 09, 2008 at 02:54:43PM +0200, Louis Rilling wrote:
> > Following an intuition, I just found a deadlock resulting from the whole default
> > groups tree locking in configfs_detach_prep().
> 
> 	Ugh, thanks for catching this :-(
>  
> > The issue here is that the VFS locks the i_mutex of the source and target
> > directories of the rename in source -> target order (because none is ascendent
> > of the other one), while configfs_detach_prep() takes them in default group
> > order (or reverse order, I'm not sure), following the order specified by the
> > groups' creator.
> 
> 	What actual targets are you renaming?  Sibling default groups?

Actually the operation tries to rename a file in the source default group
"foo/heartbeat/" to a new entry "bar" in a sibling default group "foo/node/" of the
source default group. The operation itself is silly regarding configfs
semantics, but VFS cannot know before locking the source and target
directories...

> 
> > The VFS protects itself against deadlocks of two concurrent renames with
> > interverted source and target directories with i_sb->s_vfs_rename_mutex. Perhaps
> > configfs should use the same lock before calling configfs_detach_prep()?
> > Or maybe configfs would better find an alternative to locking the whole
> > default groups tree? I strongly advocate for the latter, since this could also
> > solve our issues with lockdep ;)
> 
> 	I think the former actually works nicely.  We are playing with
> the subtree, and want to keep all operations out of it.  Except, of
> course, that we come into rmdir() with our parent i_mutex taken, so that
> violates the ordering of the rename locks, right?

Right.
I suggested to use i_sb->s_vfs_rename_mutex, but we cannot do this from
inside configfs_rmdir(), because locking an i_mutex (as the VFS does
before calling configfs_rmdir()) before s_vfs_rename_mutex will also
deadlock with lock_rename().

> 	I'm not against the latter AT ALL.  I just haven't come up with
> it yet - we can't remove parts of the tree, it must be all or none.
> Hence, we lock them all speculatively.

I'm slowly thinking about a solution, but I don't know the VFS enough
yet, especially regarding dentry invalidation and locking. Would it be possible
to start an rmdir() by moving the group to some unreachable place? We could
probably use the mutex of the configfs subsystem and enlarge its scope to
protect against concurrent mkdir(), check for user-created items under the
group (and descendent default groups) to remove, invalidate all dentries and
actually remove the group. Do you think that something is feasible in this way?

Louis

-- 
Dr Louis Rilling			Kerlabs - IRISA
Skype: louis.rilling			Campus Universitaire de Beaulieu
Phone: (+33|0) 2 99 84 71 52		Avenue du General Leclerc
Fax: (+33|0) 2 99 84 71 71		35042 Rennes CEDEX - France
http://www.kerlabs.com/
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