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

From: Arne Jansen
Date: Mon Jun 06 2011 - 02:56:47 EST

On 05.06.2011 22:15, Arne Jansen wrote:
> 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)
>> {
>> ...
>> 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.
> 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...

I just let it dump the locks in debug_show_all_locks, even though
for some reason debug_locks is false. Don't know if the result is
helpful in any way, as it might well be inaccurate.

INFO: lockdep is turned off.

Showing all locks held in the system:
2 locks held by syslog-ng/21624:
#0: (&tty->atomic_write_lock){+.+.+.}, at: [<ffffffff8142ade3>]
#1: (&tty->output_lock){+.+...}, at: [<ffffffff8142ee7a>]
1 lock held by agetty/22174:
#0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff8142fb86>]
1 lock held by agetty/22175:
#0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff8142fb86>]
1 lock held by agetty/22176:
#0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff8142fb86>]
1 lock held by agetty/22177:
#0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff8142fb86>]
1 lock held by agetty/22178:
#0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff8142fb86>]
1 lock held by agetty/22179:
#0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff8142fb86>]
1 lock held by agetty/22180:
#0: (&tty->atomic_read_lock){+.+...}, at: [<ffffffff8142fb86>]
1 lock held by tail/22197:
#0: (&rq->lock){-.-.-.}, at: [<ffffffff8185ae42>] schedule+0xe2/0x940

Some more fact that might help figure out what happens:
- I nearly always either see all 10000 messages or only 10.
Never 9, never 11. I saw 40 once, and once 190.
- If I printk only 1000 lines instead of 10000, nothing bad happens
- If /var/log/syslog is not filled with binary garbage, I also just
see the 10 lines.

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