linux-kernel - Re: [cpuops cmpxchg double V2 1/4] Generic support for this_cpu_cmpxchg

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Date:	Sun, 09 Jan 2011 10:33:43 +0200
From:	Pekka Enberg <penberg@...nel.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Christoph Lameter <cl@...ux.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"H. Peter Anvin" <hpa@...or.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [cpuops cmpxchg double V2 1/4] Generic support for
 this_cpu_cmpxchg_double

On Sat, 2011-01-08 at 12:24 -0500, Tejun Heo wrote:
> Hello,
> 
> On Fri, Jan 07, 2011 at 12:41:58PM -0600, Christoph Lameter wrote:
> > On Fri, 7 Jan 2011, Mathieu Desnoyers wrote:
> > 
> > > I have to admit that I also hate the current interface for two reasons:
> 
> Call me weird but I like this one than others.  It sure is ugly but
> the operation itself isn't a particularly pretty so it kinda matches.
> Also, this one is the least error prone and more consistent with other
> cpu ops.
> 
> > > b) the loss of the value read (the fact that the only current user of this API
> > >    does not need the value returned seems like a very weak argument to define an
> > >    API).
> > 
> > The other user of cmpxchg_double that I have in my tree also does not have
> > the need. Repeatability is not as simple to implement as with a single
> > word cmpxchg.
> 
> Yeah, even in regular cmpxchg, the read value on failure isn't of very
> high value.  The cpu usually has to go retry anyway && likely to have
> the cacheline already, so it's not gonna cost much.
> 
> So, yeah, of the proposed ones, this is my favorite.  Peter and
> Mathieu don't like it.  What do others think?  Pekka, Eric, Andrew,
> what do you guys think?

I'm not a huge fan of the API either but like you, I like it best from
the proposed ones.

			Pekka

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