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

[Ingo Molnar]

* 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)
{
...
        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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/