Re: [debug patch] printk: Add a printk killswitch to robustify NMIwatchdog messages

From: Arne Jansen
Date: Sun Jun 05 2011 - 16:16:37 EST

On 05.06.2011 21:44, Ingo Molnar wrote:

* Arne Jansen<lists@xxxxxxxxxxxxxx> 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)

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.

Can lockdep just get confused by the lockdep_off/on calls in printk
while scheduling is allowed? There aren't many users of lockdep_off().

I'll can try again tomorrow to get a dump of all logs from the
watchdog, but enough for today...

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