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

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Message-ID: <20080123133149.GA4059@ucw.cz>
Date:	Wed, 23 Jan 2008 13:31:49 +0000
From:	Pavel Machek <pavel@....cz>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...nvz.org>,
	Kirill Korotaev <dev@...ru>
Subject: Re: [PATCH] printk deadlocks if called with runqueue lock held

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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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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