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

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Date:	Mon, 6 Jun 2011 20:07:33 +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


* 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.

   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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