linux-kernel - Re: [this_cpu_xx V6 3/7] Use this

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Message-ID: <4AD53022.2050309@kernel.org>
Date:	Wed, 14 Oct 2009 10:57:54 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	Mel Gorman <mel@....ul.ie>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Subject: Re: [this_cpu_xx V6 3/7] Use this_cpu operations in slub

Hello, Christoph.

Christoph Lameter wrote:
>> Yeah, widespread use of underscored versions isn't very desirable.
>> The underscored versions should notify certain specific exceptional
>> conditions instead of being used as general optimization (which
>> doesn't make much sense after all as the optimization is only
>> meaningful with debug option turned on).  Are you interested in doing
>> a sweeping patch to drop underscores from __this_cpu_*() conversions?
> 
> Nope. __this_cpu_add/dec cannot be converted.

Right.

> __this_cpu_ptr could be converted to this_cpu_ptr but I think the __ are
> useful there too to show that we are in a preempt section.

That doesn't make much sense.  __ for this_cpu_ptr() means "bypass
sanity check, we're knowingly violating the required conditions" not
"we know sanity checks will pass here".

> The calls to raw_smp_processor_id and smp_processor_id() are only useful
> in the fallback case. There is no need for those if the arch has a way to
> provide the current percpu offset. So we in effect have two meanings of __
> right now.
> 
> 1. We do not care about the preempt state (thus we call
> raw_smp_processor_id so that the preempt state does not trigger)
> 
> 2. We do not need to disable preempt before the operation.
> 
> __this_cpu_ptr only implies 1. __this_cpu_add uses 1 and 2.

Yeah, we need to clean it up.  The naming is too confusing.

Thanks.

-- 
tejun
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