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

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Message-ID: <20061208190403.GH31068@flint.arm.linux.org.uk>
Date:	Fri, 8 Dec 2006 19:04:03 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	David Howells <dhowells@...hat.com>,
	Christoph Lameter <clameter@....com>,
	Nick Piggin <nickpiggin@...oo.com.au>, 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

On Fri, Dec 08, 2006 at 10:46:17AM -0800, Linus Torvalds wrote:
> 
> 
> On Fri, 8 Dec 2006, David Howells wrote:
> > 
> > In fact I think more things have LL/SC than have CMPXCHG.
> 
> But you cannot expose ll/sc to C, so that's a bogus argument.

Yes you can.  Well, you can on ARM at least.  Between the load exclusive
you can do anything you like until you hit the store exclusive.  If you
haven't touched the location (with anything other than another load
exclusive) and no other CPU has issued a load exclusive, your store
exclusive succeeds.  Whether you have branches, loads or stores to
other locations, etc.  None of that matters.

Not so bogus as you make out.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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