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

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Date:	Fri, 24 Dec 2010 22:53:55 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	"H. Peter Anvin" <hpa@...or.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 Thu, 23 Dec 2010, H. Peter Anvin wrote:

> 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.

Oh gosh. So we would be using a tight corner case for gcc that may only
work with certain versions of gcc? Note that the current version does only
return a boolean. There is no need for returning double words. I'd be
happy if we could *pass* double words.

> > 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.

I think then we better leave it as is. The use case so far is minimal so
we could easily change that if we get to a better solution later.
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