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

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Date:	Fri, 8 Dec 2006 09:23:57 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Russell King <rmk+lkml@....linux.org.uk>
cc:	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>, 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 Fri, 8 Dec 2006, Russell King wrote:

> As proven previously the reverse is also true.  And as shown previously
> the cheaper out of the two for all platforms is the LL/SC based
> implementation, where the architecture specific implementation can
> be _either_ LL/SC based or cmpxchg based depending on what is
> supported in their hardware.

As also shown in this thread: There are restrictions on what you can do 
between ll/sc. You would not want to use C code there. ll/sc is an thing 
that needs to be restricted to asm code. So this is not a viable proposal 
at all. ll/sc is useful to construct various atomic functions but cannot 
be directly used in C code. cmpxchg can be effectively realized using 
ll/sc.
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