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

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Date:	Tue, 20 May 2008 15:13:41 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Joel Becker <Joel.Becker@...cle.com>
Cc:	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, 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

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