Messages in this thread | | | Date | Wed, 6 Dec 2006 11:47:40 -0800 (PST) | From | Christoph Lameter <> | Subject | Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it |
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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.
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