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Subject: Re: 2.6.24-rc7 lockdep warning when poweroff
From: Peter Zijlstra <peterz@infradead.org>
To: Johannes Berg <johannes@sipsolutions.net>
Cc: Dave Young <hidave.darkstar@gmail.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Ingo Molnar <mingo@elte.hu>
In-Reply-To: <1200306902.5887.39.camel@johannes.berg>
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	 <1200302644.7415.6.camel@twins>  <1200306902.5887.39.camel@johannes.berg>
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Date: Mon, 14 Jan 2008 11:41:28 +0100
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On Mon, 2008-01-14 at 11:35 +0100, Johannes Berg wrote:
> > > When I "halt -p", lockdep warnings triggered as following (hand copy):
> > > 
> > > WARNING : at kernel/lockdep.c: 700 lookup_lock_class()
> > > 
> > > <snip>
> > > lock_acquire
> > > cleanup_workqueue_thread
> > > workqueue_cpu_callback
> > > notifier_call_chain
> > > __raw_notifier_call_chain
> > > raw_notifier_call_chain
> > > __cpu_down
> > > disable_nonboot_cpus
> > > kernel_power_off
> > > sys_reboot
> > > <snip>
> > 
> > My first guess would be a __create_workqueue_key() where a key is
> > re-used with a different workqueue name.
> 
> Hm, can you elaborate how you got there from the trace above? I don't
> think I'm following.

The warning that triggered (lockdep.c:700) means that one class (key)
was used with more than one name.

Looking at cleanup_workqueue_thread(), the lock_acquire() there works on
wq->lockdep_map, and that is only initialized at one spot:
__create_workqueue_key(), thus it stands to reason that that was
mis-used.



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