Re: spin_unlock optimization(i386)

Ingo Molnar (mingo@chiara.csoma.elte.hu)
Sat, 27 Nov 1999 00:57:48 +0100 (CET)

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in fact it's the overflow situation that triggers a causality violation
even on dual boxes. Code attached, it produces:

hell:~> ./causal 0 2
<78266> data3:0 < data4:100001
<thread1> BROKE causality! Weakly ordered memory?

but data4 is always kept smaller than data3! both data4 and data3 are only
ever modified by one of the CPUs, and this is observed on another CPU. The
above shows exactly the case we were worried about:

}
if (i > OVERFLOW) {
data4 = 0;
data3 = 0;

but the second store showed up on the other CPU before the first store.

[if 'causal 1 2' is used (which variant uses the LOCK prefix) then the
assert doesnt seem to trigger.]

i think part of the picture is that the above 'overflow branch' surely is
incorrectly predicted when it triggers. This might open up the window.

{ unless my code is buggy ;) }

Ingo

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