From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [PATCH -V3 01/11] percpu_counters: make fbc->count read atomic
	on 32 bit architecture
Date: Thu, 28 Aug 2008 09:57:37 +0200
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To: Andrew Morton <akpm@linux-foundation.org>
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On Wed, 2008-08-27 at 14:22 -0700, Andrew Morton wrote:

> btw, what the heck is percpu_counter_init_irq()?  Some mysterious
> lockdep-specific thing?
> 
> <does git-fiddle.  Oh.  crappy changelog.>
> 
> I let that one leak through uncommented.  Must be getting old. 
> Probably it will need an EXPORT_SYMBOL() sometime.

Basically all it does it break the percpu_counter lock into two classes.

One for the irq-unsafe users and one for the irq-safe users. Without
this lockdep goes splat complaining about irq recursion deadlocks and
the like between these two separate users.