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

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Message-ID: <20061208194357.GJ31068@flint.arm.linux.org.uk>
Date:	Fri, 8 Dec 2006 19:43:57 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Christoph Lameter <clameter@....com>,
	David Howells <dhowells@...hat.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 11:37:45AM -0800, Linus Torvalds wrote:
> 
> 
> On Fri, 8 Dec 2006, Russell King wrote:
> > 
> > I utterly disagree.  I could code atomic_add() as:
> 
> Sure. And Alpha could do that too. If you write the C code a specific way, 
> you can make it work. That does NOT mean that you can expose it widely as 
> a portable interface - it's still just a very _nonportable_ interface that 
> you use internally within one architecture to implement other interfaces.

However, nothing stops you wrapping the non-portable nature of ll/sc up
into the store part though.

If you can efficiently implement cmpxchg inside an ll/sc based portable
interface (yes you can) and you can implement problematical ll/sc
structures inside a cmpxchg() interface, you can do it either way around.
Only one way doesn't penalise broken ll/sc based implementations though.

That is the essence of my argument.

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