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

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Message-ID: <20090126144313.GC4445@elte.hu>
Date:	Mon, 26 Jan 2009 15:43:13 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alexey Zaytsev <alexey.zaytsev@...il.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	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


* Alexey Zaytsev <alexey.zaytsev@...il.com> wrote:

> On Wed, Jan 14, 2009 at 05:00, Nick Piggin <npiggin@...e.de> wrote:
> > 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?
> >
> 
> Hi again.
> 
> Finally got to debug this. The preempt count on the first __do_softirq entry
> ever is 0, as it is set in irq_ctx_init(). The interrupted swapper
> thread happens
> to be in the kernel_locked() state at the moment, so the warning.
> 
> I don't understand why the softirq preempt count is initialized to 0. 
> Should not it be SOFTIRQ_OFFSET instead?

hm, indeed. So this triggers on irqstacks, if an irq happens to hit the 
first time a softirq executes (ever)? After that point the preempt_count 
in the irq-stack ought to stay elevated.

	Ingo
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