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

Christoph Lameter
2006-12-06T19:48:51Z

On Wed, 6 Dec 2006, Matthew Wilcox wrote:

> 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:

Nope this is a UP implementation. There is no cpu B.