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

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Message-ID: <4DEB6F3A.3000109@die-jansens.de>
Date:	Sun, 05 Jun 2011 13:57:46 +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 13:36, Ingo Molnar wrote:
>
> * Ingo Molnar<mingo@...e.hu>  wrote:
>
>>> If the ticks stop this suggests a lockup within the printk code.
>>> [...]
>>
>> In which case the printk-killswitch patch below (to be applied
>> *instead* of the previous debugging patch i sent) should provide
>> the desired NMI watchdog output on the serial console.
>>
>> Warning: it's entirely untested.

How is the output supposed to come through? shouldn't printk revert
to early_printk instead of just returning?


>
> Note, since this is an SMP box, if the lockup messages show up with
> this patch but are mixed up with each other then adding a spinlock
> around the WARN() would probably help keeping the output serialized.
>
> A simple:
>
>   static DEFINE_SPINLOCK(watchdog_output_lock);
>
>   ...
> 	spin_lock(&watchdog_output_lock);
>   ...
> 	[ the WARN_ON() logic. ]
>   ...
> 	spin_unlock(&watchdog_output_lock);
>   ...
>
> would suffice.
>
> Thanks,
>
> 	Ingo
> --
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