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

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Message-ID: <4AD6D3A0.6040706@kernel.org>
Date:	Thu, 15 Oct 2009 16:47:44 +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

Christoph Lameter wrote:
> On Wed, 14 Oct 2009, Tejun Heo wrote:
> 
>>> __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".
> 
> Are you defining what __ means for this_cpu_ptr?

I was basically stating the different between raw_smp_processor_id()
and smp_processor_id() which I thought applied the same to
__this_cpu_ptr() and this_cpu_ptr().

>>> 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.
> 
> Its consistent if __ means both 1 and 2. If we want to distinguish it then
> we may want to create raw_this_cpu_xx which means that we do not call
> smp_processor_id() on fallback but raw_smp_processor_id(). Does not
> matter if the arch provides a per cpu offset.
> 
> This would mean duplicating all the macros. The use of raw_this_cpu_xx
> should be rare so maybe the best approach is to say that __ means only
> that the macro does not need to disable preempt but it still checks for
> preemption being off. Then audit the __this_cpu_xx uses and see if there
> are any that require a raw_ variant.
> 
> The vm event counters require both no check and no preempt since they can
> be implemented in a racy way.

The biggest grief I have is that the meaning of __ is different among
different accessors.  If that can be cleared up, we would be in much
better shape without adding any extra macros.  Can we just remove all
__'s and use meaningful pre or suffixes like raw or irq or whatever?

Thanks.

-- 
tejun
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