linux-kernel - Re: [RFC][PATCH 0/3] configfs: Make nested default groups lockdep-friendly

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Date:	Wed, 21 May 2008 11:23:04 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Joel Becker <Joel.Becker@...cle.com>,
	Louis Rilling <Louis.Rilling@...labs.com>,
	linux-kernel@...r.kernel.org, ocfs2-devel@....oracle.com
Subject: Re: [RFC][PATCH 0/3] configfs: Make nested default groups
	lockdep-friendly

On Tue, 2008-05-20 at 15:13 -0700, Arjan van de Ven wrote:
> On Tue, 20 May 2008 14:56:39 -0700
> Joel Becker <Joel.Becker@...cle.com> wrote:
> 
> > On Tue, May 20, 2008 at 09:58:10AM -0700, Arjan van de Ven wrote:
> > > On Tue, 20 May 2008 18:33:20 +0200
> > > Louis Rilling <Louis.Rilling@...labs.com> wrote:
> > > 
> > > > The following patches fix lockdep warnings resulting from
> > > > (correct) recursive locking in configfs.
> > > >
> > > > ...
> > > >
> > > > Since lockdep does not handle such correct recursion, the idea is
> > > > to insert lockdep_off()/lockdep_on() for inode mutexes as soon as
> > > > the level of recursion of the I_MUTEX_PARENT -> I_MUTEX_CHILD
> > > > dependency pattern increases.
> > > 
> > > I'm... not entirely happy with such a solution ;(
> > > 
> > > there must be a better one.
> > 
> > 	We're trying to find it.  I really appreciate Louis taking the
> > time to approach the issue.  His first pass was to add 1 to
> > MUTEX_CHILD for each level of recursion.  This has a very tight limit
> > (4 or 5 levels), but probably covers all users that exist and perhaps
> > all that ever will exist.  However, it means passing the lockdep
> > annotation level throughout the entire call chain across multiple
> > files.  It was definitely less readable.
> > 	This approach takes a different tack - it's very readable, but
> > it assumes that the currently correct locking will always remain so -
> > a particular invariant that lockdep exists to verify :-)
> > 	Louis, what about sticking the recursion level on
> > configfs_dirent?  That is, you could add sd->s_level and then use it
> > when needed.  THis would hopefully avoid having to pass the level as
> > an argument to every function.  Then we can go back to your original
> > scheme.  If they recurse too much and hit the lockdep limit, just
> > rewind everything and return -ELOOP.
> 
> you can also make a new lockdep key for each level... not pretty but it
> works

Yeah, that is what I've done in the past for trees:

http://programming.kicks-ass.net/kernel-patches/concurrent-pagecache/23-rc1-rt/radix-concurrent-lockdep.patch



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