linux-kernel - Re: [thiscpuops upgrade 08/10] percpu: generic this_cpu_cmpxchg() and this_cpu_cmpxchg

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Message-ID: <4CEFE71F.5090303@kernel.org>
Date:	Fri, 26 Nov 2010 17:58:07 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Christoph Lameter <cl@...ux.com>, akpm@...ux-foundation.org,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [thiscpuops upgrade 08/10] percpu: generic this_cpu_cmpxchg()
 and this_cpu_cmpxchg_double support

On 11/26/2010 05:56 PM, Eric Dumazet wrote:
> Le vendredi 26 novembre 2010 à 17:51 +0100, Tejun Heo a écrit :
>> On 11/24/2010 12:51 AM, Christoph Lameter wrote:
>>> +/*
>>> + * cmpxchg_double replaces two adjacent scalars at once. The first parameter
>>> + * passed is a percpu pointer, not a scalar like the other this_cpu
>>> + * operations. This is so because the function operates on two scalars
>>> + * (must be of same size). A truth value is returned to indicate success or
>>> + * failure (since a double register result is difficult to handle).
>>> + * There is very limited hardware support for these operations. So only certain
>>> + * sizes may work.
>>> + */
>>> +#define __this_cpu_generic_cmpxchg_double(pcp, oval1, oval2, nval1, nval2)	\
>>
>> Ah... this is scary.  If it proves to be useful enough, sure, but
>> otherwise I'd like to avoid it.
> 
> This is mandatory for several lockless algos, dont be afraid, its a
> single instruction ;)

Thanks for holding my hands. :-)

What I'm afraid of is generic code switching to use it in very hot
paths when a lot of archs can't actually do it leading to performance
regressions.  Is this something which is available on most archs?

Thanks.

-- 
tejun
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