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    SubjectRe: spin_unlock optimization(i386)
    Date
    "Ingo Molnar wrote:"
    >
    > the correct one is:
    >
    > CPU0 CPU1
    >
    > for (;;) { for (;;) {
    > i++; b = j;
    > j++; a = i;
    > } if (a < b)
    > BUG();
    > }

    Just to confirm what you are saying:

    invariant: j <= i <= j+1 invariant: b <= a

    and this test can indeed never be activated. But ...


    > the 'symmetric' test:
    >
    > CPU0 CPU1
    >
    > for (;;) { for (;;) {
    > i1++; i2++;
    > j1++; j2++;
    > b = j2; b = j1;
    > a = i2; a = i1;
    > if (a < b) if (a < b)
    > BUG(); BUG();
    > } }

    invariant: j1 <= i1 <= j1+1 invariant: j2 <= i2 <= j2+1

    But that's all! You can get

    i1= 50 i2= 100
    j1= 50 j2= 100
    . b = 50
    b = 100 .
    . a = 50
    . TEST
    a = 100 .


    which then tests with a < b on the right. I think ... (this gets
    confusing).



    Peter

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