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

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Message-ID: <Pine.LNX.4.64.0612061128340.27363@schroedinger.engr.sgi.com>
Date:	Wed, 6 Dec 2006 11:29:42 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Matthew Wilcox <matthew@....cx>
cc:	Linus Torvalds <torvalds@...l.org>,
	Russell King <rmk+lkml@....linux.org.uk>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...l.org>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	Linux Kernel Mailing List <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, Matthew Wilcox wrote:

> It's just been pointed out to me that the parisc one isn't safe.
> 
> <dhowells> imagine variable X is set to 3
> <dhowells> CPU A issues cmpxchg(&X, 3, 5)
> <dhowells> you'd expect that to change X to 5
> <dhowells> but what if CPU B assigns 6 to X between cmpxchg reading X
> and it setting X?

The same could happen with a regular cmpxchg. Cmpxchg changes it to 5 and 
then other cpu performs a store before the next instruction.

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