linux-kernel - Re: [cpuops cmpxchg double V1 1/4] Generic support for this_cpu_cmpxchg

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Message-ID: <4D11373D.9010205@zytor.com>
Date:	Tue, 21 Dec 2010 15:24:45 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	Tejun Heo <tj@...nel.org>, akpm@...ux-foundation.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [cpuops cmpxchg double V1 1/4] Generic support for this_cpu_cmpxchg_double

On 12/21/2010 02:36 PM, Christoph Lameter wrote:
> On Sat, 18 Dec 2010, Tejun Heo wrote:
> 
>> pointers from the beginning.  Anyways, that's too late, so is it
>> completely impossible to make cmpxchg_double's take a scalar value?
>> It can take the pointer all the same, no?
> 
> It could take a scalar value like the others but we are then not operating
> on the scalar alone but also on the following field.
> 

I'm a bit confused on this one.  The standard cmpxchg() takes a scalar
and a pointer, and returns a scalar.  The equivalent for the "double"
variety would be to return a compound object, basically:

struct double_ulong {
	unsigned long v[2];
};

... which can be returned in registers on both i386 and x86-64.

It's a bit clumsy from a type perspective, but I'm not sure that that is
a bad thing.  Doing too much type genericity has caused us problems in
the past.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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