linux-kernel - Re: next-20090107: WARNING: at kernel/sched.c:4435 sub_preempt

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Message-ID: <20090114020040.GA19806@wotan.suse.de>
Date:	Wed, 14 Jan 2009 03:00:40 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Alexey Zaytsev <alexey.zaytsev@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Laurent Riffard <laurent.riffard@...e.fr>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: next-20090107: WARNING: at kernel/sched.c:4435 sub_preempt_count

On Sun, Jan 11, 2009 at 03:49:45AM +0100, Ingo Molnar wrote:
> 
> * Alexey Zaytsev <alexey.zaytsev@...il.com> wrote:
> 
> > One more instance of http://marc.info/?l=linux-kernel&m=123134586202636&w=2
> > Added Ingo Molnar to CC.
> 
> added Nick on Cc:. Nick, it's about:
> 
> > commit 7317d7b87edb41a9135e30be1ec3f7ef817c53dd
> > Author: Nick Piggin <nickpiggin@...oo.com.au>
> > Date:   Tue Sep 30 20:50:27 2008 +1000
> >
> >    sched: improve preempt debugging
> 
> causing a seemingly spurious warning.

I don't know how it is spurious... Presumably the sequence _would_ have
caused preempt count to go negative if the bkl were not held...

__do_softirq does a __local_bh_disable on entry, and it seems like the
_local_bh_enable on exit is what causes this warning. So something is
unbalanced somehow. Or is it some weird thing we do in early boot that
I am missing?

Can you put in some printks around these functions in early boot to
get an idea of what preempt_count is doing?



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