Date: Sun, 5 Jun 2011 21:44:19 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Arne Jansen <lists@die-jansens.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
        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
Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI
 watchdog messages
Message-ID: <20110605194419.GA12965@elte.hu>
References: <20110605151323.GA30590@elte.hu>
 <20110605152641.GA31124@elte.hu>
 <20110605153218.GA31471@elte.hu>
 <4DEBA9CC.4090503@die-jansens.de>
 <4DEBB05C.8090506@die-jansens.de>
 <4DEBB3DA.8060001@die-jansens.de>
 <20110605172052.GA1036@elte.hu>
 <4DEBBFF9.2030101@die-jansens.de>
 <20110605185957.GA3452@elte.hu>
 <4DEBD95B.6030901@die-jansens.de>
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* Arne Jansen <lists@die-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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