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

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Message-ID: <20061206193641.GY3013@parisc-linux.org>
Date:	Wed, 6 Dec 2006 12:36:41 -0700
From:	Matthew Wilcox <matthew@....cx>
To:	Christoph Lameter <clameter@....com>
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, Dec 06, 2006 at 11:29:42AM -0800, Christoph Lameter wrote:
> 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.

For someone who's advocating use of cmpxchg, it seems you don't
understand its semantics!  In the scenario dhowells pointed out, X would
be left set to 5.  X should have the value 6 under any legitimate
implementation:

CPU A		CPU B
cmpxchg(3,5)
		X = 6

CPU A		CPU B
		X = 6
cmpxhcg(3,5)

CPU A
cmpxchg(3,
		X = 6
5)

Given that even yourself got confused about how to use it, perhaps it's
not a good idea to expose this primitive to most programmers anyway?
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