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SubjectRe: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it
On Fri, Dec 08, 2006 at 09:06:00AM -0800, Christoph Lameter wrote:
> On Fri, 8 Dec 2006, David Howells wrote:
> > > It is the most universal atomic instruction that I know of.
> >
> > I think TAS-type things and XCHG-type things are more common.
> Huh? The most popular architectures are i386 x86_64 sparc ia64 etc which
> all have one or the other form of cmpxchg (some issues with early sparc
> and i386).

According to the latest figures, 621 million ARM processors were
shipped in Q3, which equates to about 78 processors per second.
(taken from

I'd like to know the figures for these other so-called "popular

But in any case that's an utterly irrelevant point to the argument
at hand.

> > In fact I think more things have LL/SC than have CMPXCHG.
> LL/SC can be easily used to come up with a cmpxchg equivalent.

As proven previously the reverse is also true. And as shown previously
the cheaper out of the two for all platforms is the LL/SC based
implementation, where the architecture specific implementation can
be _either_ LL/SC based or cmpxchg based depending on what is
supported in their hardware.

I'm going to keep saying this until people get it.

Russell King
Linux kernel 2.6 ARM Linux -
maintainer of:
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