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

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Date:	Tue, 13 Oct 2009 11:20:57 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.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

On Tue, 13 Oct 2009, Tejun Heo 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.

__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.

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.

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