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

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Date:	Thu, 23 Dec 2010 18:16:52 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Tejun Heo <tj@...nel.org>
cc:	"H. Peter Anvin" <hpa@...or.com>, 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 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.

> > 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.




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