linux-kernel - Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it

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Date:	Fri, 08 Dec 2006 16:56:57 +0000
From:	David Howells <dhowells@...hat.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Russell King <rmk+lkml@....linux.org.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	David Howells <dhowells@...hat.com>, torvalds@...l.org,
	akpm@...l.org, linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it 

Christoph Lameter <clameter@....com> wrote:

> cmpxchg is the simplest solution to realize many other atomic operations and
> its widely available on a wide variety of platforms.

But by no means all.  Where it doesn't exist it can only be properly emulated
by something such as LL/SC if they are there.  If not, then you can't do a
safe cmpxchg on SMP.

> It is the most universal atomic instruction that I know of.

I think TAS-type things and XCHG-type things are more common.

In fact I think more things have LL/SC than have CMPXCHG.

David
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