Re: [PATCH] printk deadlocks if called with runqueue lock held

Pavel Machek
2008-01-23T18:22:58Z

Hi!

> I thought that one could place a printk anywhere without worrying.
> But it seems that it is not wise to place a printk where the runqueue
> lock is held.
>
> I just spent two hours debugging why some of my code was locking up,
> to find that the lockup was caused by some debugging printk's that
> I had in the scheduler. The printk's were only in rare paths so
> they shouldn't be too much of a problem, but after I hit the printk
> the system locked up.
>
> Thinking that it was locking up on my code I went looking down the
> wrong path. I finally found (after examining an NMI dump) that
> the lockup happened because printk was trying to wakeup the klogd
> daemon, which caused a deadlock when the try_to_wakeup code tries
> to grab the runqueue lock.

Could try_to_wakeup use trylock, and only avoid wakeup if lock is
already held?
Pavel
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