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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Message-ID: <20080611211523.GB14754@mail.oracle.com>
Date:	Wed, 11 Jun 2008 14:15:23 -0700
From:	Joel Becker <Joel.Becker@...cle.com>
To:	Louis Rilling <Louis.Rilling@...labs.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 Wed, Jun 11, 2008 at 07:30:47PM +0200, Louis Rilling wrote:
> I've spoken too fast. Doing things in d_revalidate() (or even in
> configfs_lookup()) is just too early: after they are called and before
> lock_rename(), nothing can prevent rmdir() from being called.

	Yeah, that's what I was saying.

> I'm afraid that we need a way to get rid of the whole tree locking in
> configfs_detach_prep(). Imagine that we protect all calls to configfs_rmdir()
> and configfs_mkdir() with a global mutex (say configfs_dir_mutex). Is it still
> needed to take default group's i_mutex in configfs_detach_prep() to check for
> the validity of the rmdir()?

	That's not a bad idea.  I was basically sure we need the tree
locking to prevent other VFS operations from happening, but the more I
think about it, the only reason we do the prep locking is to check and
protect the ENOTEMPTY case.  I've bounced it off Mark too, and he
agrees.

> 	After this check, we could set a REMOVING flag in configfs_dirent of all
> default groups (could actually be done by configfs_detach_prep(), making
> configfs_detach_rollback() resetting the flag), release configfs_dir_mutex, and
> then call configfs_detach_group(), with detach_groups() taking default group's
> i_mutex right before calling configfs_detach_group() recursively (this would
> lead to the same path locking as in populate_groups() or
> configfs_depend_prep()).

	We already set the flag - CONFIGFS_USET_DROPPING.  The only
change is that you don't take i_mutex anymore.  For the static mutex,
the name 'configfs_dir_mutex' is fine.

> 	On mkdir() side, once configfs_dir_mutex is acquired, we first check the
> REMOVING flag, and proceed if it is not set.

	And if it is set, we return -ENOENT.

> Just tell me if you think that it's feasible, and I'll send you a patch if it's

	Have at!  Thanks.

Joel

-- 

"Vote early and vote often." 
        - Al Capone

Joel Becker
Principal Software Developer
Oracle
E-mail: joel.becker@...cle.com
Phone: (650) 506-8127
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