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

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Date:	Sun, 5 Jun 2011 21:44:19 +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:

> From the timing I see I'd guess it has something to do with the 
> scheduler kicking in during printk. I'm neither familiar with the 
> printk code nor with the scheduler.

Yeah, that's the well-known wake-up of klogd:

void console_unlock(void)
{
...
        up(&console_sem);

actually ... that's not the klogd wake-up at all (!). I so suck today 
at bug analysis :-)

It's the console lock()/unlock() sequence, and guess what does it:

 drivers/tty/tty_io.c:   console_lock();
 drivers/tty/vt/selection.c:     console_lock();

and the vt.c code in a dozen places.

So maybe it's some sort of tty related memory corruption that was 
made *visible* via the extra assert that the scheduler is doing? The 
pi_list is embedded in task struct.

This would explain why only printk() triggers it and other wakeup 
patterns not.

Now, i don't really like this theory either. Why is there no other 
type of corruption? And exactly why did only the task_struct::pi_lock 
field get corrupted while nearby fields not? Also, none of the fields 
near pi_lock are even remotely tty related.

Thanks,

	Ingo
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