lkml.org 
[lkml]   [2010]   [Dec]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [cpuops cmpxchg double V1 1/4] Generic support for this_cpu_cmpxchg_double
On 12/23/2010 04:16 PM, Christoph Lameter wrote:
> On Wed, 22 Dec 2010, Tejun Heo wrote:
>
>>> 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.
>
> Really? How would that work? I tried with uint128 but could not get the
> compiler to do the right thing.
>

There are two return registers; two machine registers can be returned in
registers. [u]int128 is poorly implemented in a lot of gcc versions,
since it really hasn't been exercised. However, two-word structures
should work. I do not believe a two-word *array* works, though.

>>> 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.
>>
>> Yeah, the above might be better too. Is there any reason to use
>> cmpxchg_double on anything smaller?
>
> Yes. You may want to use cmpxchg_double on 32 bit entities for backwards
> compatibilities sake or any other smaller unit size. But those could also
> be realized using this_cpu_cmpxchg_<double the size> by just aggregating
> the amount.
>
> If we can indeed pass 128 bit entities (as claimed by hpa) via registers
> then the logical choice would be to do
>
> this_cpu_cmpxchg_16(pcp, old, new)
>
> instead of cmpxchg_double. All parameters would have to be bit.
> Then we can avoid the strange cmpxchg_double semantics and can completely
> avoid introducing those.

I'm not sure it works with passing in a structure.

-hpa


\
 
 \ /
  Last update: 2010-12-24 01:27    [W:0.083 / U:0.364 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site