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

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Date:	Sun, 5 Jun 2011 13:36:27 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Arne Jansen <lists@...-jansens.de>
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


* 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.

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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