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    SubjectRe: [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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