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

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Date:	Mon, 06 Jun 2011 18:50:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Arne Jansen <lists@...-jansens.de>,
	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 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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