Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI
 watchdog messages
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Arne Jansen <lists@die-jansens.de>,
        Linus Torvalds <torvalds@linux-foundation.org>, mingo@redhat.com,
        hpa@zytor.com, linux-kernel@vger.kernel.org, efault@gmx.de,
        npiggin@kernel.dk, akpm@linux-foundation.org, frank.rowand@am.sony.com,
        tglx@linutronix.de, linux-tip-commits@vger.kernel.org
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Date: Mon, 06 Jun 2011 18:50:48 +0200
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On Mon, 2011-06-06 at 18:44 +0200, Peter Zijlstra wrote:
> +void printk_tick(void)
> +{
> +       if (!__this_cpu_read(printk_pending))
> +               return;
> +
> +       /*
> +        * Try to acquire and then immediately release the
> +        * console semaphore. The release will do all the
> +        * actual magic (print out buffers, wake up klogd,
> +        * etc). 
> +        */
> +       if (console_trylock_for_printk(smp_processor_id())) {
> +               console_unlock();
> +               __this_cpu_write(printk_pending, 0);
> +       }
> +} 

Aside from not compiling (someone stuck a ref to wake_up_klogd somewhere
in lib/) this does delay the whole of printk() output by up to a jiffy,
if the machine dies funny you could be missing large parts of the
output :/


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