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Date: Sun, 5 Jun 2011 13:36:27 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Arne Jansen <lists@die-jansens.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>, mingo@redhat.com,
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        npiggin@kernel.dk, akpm@linux-foundation.org, frank.rowand@am.sony.com,
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Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI
 watchdog messages
Message-ID: <20110605113627.GA25724@elte.hu>
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* Ingo Molnar <mingo@elte.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