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

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Date:	Mon, 06 Jun 2011 20:14:21 +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 06.06.2011 20:07, Ingo Molnar wrote:
>
> * Arne Jansen<lists@...-jansens.de>  wrote:
>
>> On 06.06.2011 19:11, Ingo Molnar wrote:
>>>
>>> * Peter Zijlstra<peterz@...radead.org>   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);
>>>> +	}
>>>> +}
>>>
>>> Arne does this fix the hang you are seeing?
>>
>> What do you want me to test? just replace printk_tick with the
>> above version? If I do that, the machine doesn't even boot up any
>> more.
>
> Yeah.
>
> So i think we want two patches:
>
>   - The first one that minimally removes the lockdep_off()/on() dance
>     and fixes the regression: the patch that i sent earlier today.
>     I *think* that should fix the crash.

Isn't the regression just the false lockdep_assert_held(&p->pi_lock)?
The patch Peter sent earlier seems like the minimal changeset to fix
that, plus it fixes a bug that might pop up somewhere else, too.

>
>     3.0 material.
>
>   - The second one that moves console_sem wakeups to the jiffies tick.
>     It does not push the acquiring and the console->write() calls to
>     jiffies context, only delays the wakeup.
>
>     3.1 material.
>
> Thanks,
>
> 	Ingo

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