linux-kernel - Re: [RT] Lockdep warning on boot with 2.6.31-rc5-rt1.1

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Message-Id: <1249663785.32113.754.camel@twins>
Date:	Fri, 07 Aug 2009 18:49:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Clark Williams <williams@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"greg@...ah.com" <greg@...ah.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: [RT] Lockdep warning on boot with 2.6.31-rc5-rt1.1

On Fri, 2009-08-07 at 12:45 -0400, Alan Stern wrote:
> On Fri, 7 Aug 2009, Peter Zijlstra wrote:

> > The other proposal was creating a fixed list of classes and register
> > each device at a class corresponding to its depth in the tree. I can't
> > remember what was wrong with that, but I seem to have been persuaded
> > that that was hard too.
> 
> It probably would work for the most part.  However a possible scenario
> involves first locking a parent and then locking all its children.  (I
> don't know if this ever happens anywhere, but it might.)  This can't
> cause a deadlock but it would run into trouble with depth-based
> classes.

If you know which parent is locked, we can solve that with
mutex_lock_nest_lock() [ doesn't currently exist, but is analogous to
spin_lock_nest_lock() ] and together with
http://lkml.org/lkml/2009/7/23/222 that would allow you to lock up to
2048 children.

Would something like that work?

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