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On Fri, 26 Nov 1999, Erich Boleyn wrote:
> So, as I said before, IA32 isn't reordering anything in a visible way,
> it's effectively "shielding" the address on cpu1 because it hasn't
> gotten out of it's store queue yet.
hm, then why does causal.c trigger a causality violation on a 8-way box?
Interestingly it's getting triggered if there are two copies (4 threads
total) running. [which could be simply just randomness added to the system
bus load and thus statistically finds the window]
the basic causality test is what was analyzed yesterday:
CPU0 CPU1
for (;;) { for (;;) {
i++; a = j;
j++; b = i;
} if (a < b)
BUG()
}
i've extended this to do the same thing in a 'symmetric' way, still two
threads on two CPUs, but now both are doing loads and stores and the above
causality test:
CPU0 CPU1
for (;;) { for (;;) {
i1++; i2++;
j1++; j2++;
a = j2; a = j1;
b = i2; b = i1;
if (a < b) if (a < b)
BUG(); BUG();
} }
(i've attached the code again. i1, j1, i2, j2 are shared global, a, b are
private local variables.)
note that this cannot be reproduced on Andrea's dual-PII box (neither on
my dual-PIII box), but can be reproduced on my 8-way Xeon box, if two
copies of the above are running. [maybe if i added delay timings to the
above then we could trigger it on dual boxes too] 'two copies' means two
independent pairs of the above.
the code is simple and obvious enough extension of the most trivial
causality test, but it doesnt work.
Ingo
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