Messages in this thread | | | Date | Wed, 6 Dec 2006 11:29:42 -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:
> 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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