linux-kernel - Re: lockdep false positive in double_lock

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Date:	Fri, 18 May 2012 09:42:58 -0700
From:	Roland Dreier <roland@...nel.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	rostedt <rostedt@...dmis.org>
Subject: Re: lockdep false positive in double_lock_balance()?

> *phew* you actually made me think there ;-)

Sorry for that.

> The race you describe exists, except that's not how lockdep works. Both
> cpu's would have a different task (one would hope to presume) and the
> held lock stack is per task. So even if busiest_rq on cpu1 (lock case)
> is the same lock as this_rq on cpu0 (unlock case), they're in different
> stacks with different states.

Got it.  I confused myself between user processes (which might have
multiple threads on multiple CPUs) and kernel tasks.

> Something like this should fix it I think..

Thanks, I'll throw this in and let you know if we see this again.
But it's very rare so hard to know one way or another for quite
a while.

 - R.
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