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

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Date:	Wed, 6 Dec 2006 11:16:55 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Russell King <rmk+lkml@....linux.org.uk>
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, 6 Dec 2006, Russell King wrote:

> 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.

But then its also just requires disable/enable interrupts on UP which may 
be cheaper than an atomic operation.

> For CPUs with load locked + store conditional, it is expensive.

Because it locks the bus? I am not that familiar with those architectures 
but it seems that those will have a general problem anyways.

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

And thereby denying the kernel developers to use a simple atomic SMP 
operation? Adding additional defines for each arch and each performance 
critical piece of kernel logic?
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