linux-kernel - Re: [debug patch] printk: Add a printk killswitch to robustify NMI watchdog messages

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Date:	Mon, 06 Jun 2011 09:34:18 +0200
From:	Arne Jansen <lists@...-jansens.de>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	efault@....de, npiggin@...nel.dk, akpm@...ux-foundation.org,
	frank.rowand@...sony.com, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI
 watchdog messages

On 05.06.2011 17:26, Ingo Molnar wrote:
> 
> * Ingo Molnar <mingo@...e.hu> wrote:
> 
>>
>> * Arne Jansen <lists@...-jansens.de> wrote:
>>
>>> sched.c:934: in function __task_rq_lock
>>>         lockdep_assert_held(&p->pi_lock);
>>
>> Oh. Could you remove that line with the patch below - does it result 
>> in a working system?
>>
>> Now, this patch alone just removes a debugging check - but i'm not 
>> sure the debugging check is correct - we take the pi_lock in a raw 
>> way - which means it's not lockdep covered.
>>
>> So how can lockdep_assert_held() be called on it?
> 
> Ok, i'm wrong there - it's lockdep covered.
> 
> I also reviewed all the __task_rq_lock() call sites and each of them 
> has the pi_lock acquired. So unless both Peter and me are blind, the 
> other option would be some sort of memory corruption corrupting the 
> runqueue.

Another small idea, can we install the assert into a pre-0122ec5b02f766c
to see if it's an older problem that just got uncovered by the assert?

-Arne

> 
> But ... that looks so unlikely here, it's clearly heavy printk() and 
> console_sem twiddling that triggers the bug, not any other scheduler 
> activity.
> 
> Thanks,
> 
> 	Ingo

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