linux-kernel - Re: [cpuops cmpxchg double V2 1/4] Generic support for this_cpu_cmpxchg

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Date:	Fri, 21 Jan 2011 08:30:41 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Pekka Enberg <penberg@...helsinki.fi>,
	Christoph Lameter <cl@...ux.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [cpuops cmpxchg double V2 1/4] Generic support for this_cpu_cmpxchg_double

There is a technical argument: any noninline version will have actual code overhead.

"Tejun Heo" <tj@...nel.org> wrote:

>Hello, Peter.
>
>On Fri, Jan 21, 2011 at 07:31:55AM -0800, H. Peter Anvin wrote:
>> I really object to passing two pointers where one of them has to be a
>> fixed offset to the other.  That really doesn't make any sense.
>
>Yeah, I hear you, but it really comes down to which ugliness disgusts
>one the most.  That, unfortunately, is inherently very subjective when
>there's no significantly better choice.
>
>For me, the double parameter thing at least seems to have the
>advantages of being able to verify the two intended memory locations
>to be used actually are together and looking ugly reflecting its true
>nature.
>
>The inherent ugliness stems from the fact that we don't have the
>built-in data type to properly deal with this.  Array of length two
>might be better fit, but I can see as many downsides with that too.
>
>So, if anyone can give something clearly better for technical reasons,
>I'll be more than happy to take it, but as it currently stands, it
>seems we'll have to choose one among uglies and not everyone would be
>happy about the choice.  :-(
>
>Thanks.
>
>-- 
>tejun

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