linux-kernel - Re: irq lock inversion

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Date:	Fri, 06 Nov 2009 16:45:44 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Jiri Kosina <jkosina@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>, cl@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: irq lock inversion

Ingo Molnar wrote:
>>> This warning is bogus -- sched_init() is being called very early with IRQs
>>> disabled, and the irqsave/restore code paths in pcpu_alloc() are only for early
>>> init. The path can never be called from irq context once the early init
>>> finishes. Rationale for this is explained in changelog of the commit mentioned
>>> above.
>>>
>>> This problem can be encountered generally in any other early code running
>>> with IRQs off and using irqsave/irqrestore.
>>>
>>> Reported-by: Yinghai Lu <yhlu.kernel@...il.com>
>>> Signed-off-by: Jiri Kosina <jkosina@...e.cz>
>> Looks good to me.  Ingo, what do you think?
> 
> Ugh, this explanation is _BOGUS_. As i said, taking a lock with irqs 
> disabled does _NOT_ mark a lock as 'irq safe' - if it did, we'd have 
> false positives left and right.
> 
> Read the lockdep message please, consider all the backtraces it prints, 
> it says something different.

Ah... okay, the pcpu_free() path is correctly marking the lock
irqsafe.  I assumed this was caused by recent pcpu_alloc() change.
Sorry about that.  The lock inversion problem has always been there,
it just never showed up because none has use allocation map that large
I suppose.

So, the correct fix would be either 1. push down irqsafeness down to
vmalloc locks or 2. the rather ugly unlock-lock dancing in
pcpu_extend_area_map() I posted earlier.  For 2.6.32, I guess we'll
have to go with #2.  For longer term, we'll probably have to do #1 as
it's required to implement atomic percpu allocations too.

I'll try to reproduce the problem here and verify the previous locking
dance patch.

Thanks.

-- 
tejun
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