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

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Message-ID: <20061206190025.GC9959@flint.arm.linux.org.uk>
Date:	Wed, 6 Dec 2006 19:00:25 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Christoph Lameter <clameter@....com>
Cc:	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

On Wed, Dec 06, 2006 at 10:56:14AM -0800, Christoph Lameter wrote:
> I'd really appreciate a cmpxchg that is generically available for 
> all arches. It will allow lockless implementation for various performance 
> criticial portions of the kernel.

Let's recap on cmpxchg.

For CPUs with no atomic operation other than SWP, it is not lockless.
For CPUs with load locked + store conditional, it is expensive.

I've shown in the past that a generic implementation based around ll/sc
can be cheaply implemented using cmpxchg.  The reverse is *not* true.

If you want an operation for performance critical portions of the
kernel, please allow architecture maintainers the freedom to use their
best performance enhancements.

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