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

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Message-ID: <Pine.LNX.4.44L0.0908071725510.8084-100000@netrider.rowland.org>
Date:	Fri, 7 Aug 2009 17:30:45 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Peter Zijlstra <peterz@...radead.org>
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, 7 Aug 2009, Peter Zijlstra wrote:

> 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.

Not only do I know not which parent is locked, I don't even know if 
this ever happens anywhere at all!  My point was purely theoretical.

> Would something like that work?

Perhaps -- I don't understand what spin_lock_nest_lock() is supposed to
do.

Alan Stern

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