linux-kernel - Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an

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Date:	Tue, 12 Dec 2006 10:37:15 +0000
From:	David Howells <dhowells@...hat.com>
To:	linux@...izon.com
Cc:	nickpiggin@...oo.com.au, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org, torvalds@...l.org
Subject: Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an 

linux@...izon.com wrote:

> do {
> 	oldvalue = ll(addr)
> 	newvalue = ... oldvalue ...
> } while (!sc(addr, oldvalue, newvalue))
> 
> Where sc() could be a cmpxchg.  But, more importantly, if the
> architecture did implement LL/SC, it could be a "try plain SC; if
> that fails try CMPXCHG built out of LL/SC; if that fails, loop"

If sc() is implemented with cmpxchg(), then this is a very silly piece of
code.  cmpxchg() returns the current value if it fails, rendering a repetition
of ll() pointless.  In some circumstances, you should really do it by putting
the cmpxchg() up front with what you expect the most likely substitution to
be, and that then doesn't require the initial load.

David
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