linux-kernel - Re: Debugging Thinkpad T430s occasional suspend failure.

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Date:	Fri, 15 Feb 2013 09:44:35 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Jones <davej@...hat.com>, Hugh Dickins <hughd@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul McKenney <paul.mckenney@...aro.org>
Subject: Re: Debugging Thinkpad T430s occasional suspend failure.

On Thu, Feb 14, 2013 at 06:09:44PM -0800, Linus Torvalds wrote:
> On Thu, Feb 14, 2013 at 5:15 PM, Dave Jones <davej@...hat.com> wrote:
> >
> > Given I never saw this on a Fedora kernel, just my self-built ones, I eventually
> > gave up on bisecting code, and switched to bisecting config options.
> > I should have started this way, as I figured it out within an hour.
> >
> > 3.7 merge window is when I started seeing this, and here's what got introduced
> > during that time..
> >
> > commit e3ebfb96f396731ca2d0b108785d5da31b53ab00
> > Author: Paul E. McKenney <paul.mckenney@...aro.org>
> > Date:   Mon Jul 2 14:42:01 2012 -0700
> >
> >     rcu: Add PROVE_RCU_DELAY to provoke difficult races
> >
> > 'difficult' is an understatement.  This explains why some of those 'good'
> > bisects survived 100 suspends on one day, and failed the next.
> >
> > Unfortunatly, I don't think there's any sane way to retrieve whatever debug
> > info might be getting spewed.
> 
> Hmm. I have to say, that's a particularly unhelpful config option. It
> may make races much easier to hit, but when you do hit them, what's
> the symptoms of said race?
> 
> Paul? Apparently you end up with a dead machine at least during resume
> and no oops. Which isn't very helpful. Maybe there is possibly some
> BUG_ON() in the RCU code somewhere?
> 
> So Paul, if you know what the common symptoms of the bug that that
> debug option helps trigger are, is there some way to make them less
> lethal and still print out useful information?

This commit was designed to increase the probability of hitting the
races described in http://lwn.net/Articles/453002/.  These races result
in deadlocks involving the runqueue lock (and perhaps also the priority
inheritance locks).  And yes, I most certainly should have described
this in the commit message.  :-(

So it looks like Dave is hitting some other race/bug than the one that
this commit was designed to expose.

I must confess that I don't know how to proceed without any meaningful
debug information here.  Feel free to revert the above commit if you would
like -- I can of course always maintain it locally for my own testing.

							Thanx, Paul

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