On Apr 13, 2004, at 2:39 PM, Boehm, Hans wrote:
> I think I may have been (or maybe still am) confused on a similar
> point, so maybe this helps.
>
> The problem with section 7.6 seems to be that it's not immediately
> apparent that you can justify
> a sequentially consistent execution.
>
> If we have
>
> Thread 1
> x = 1;
> r1 = x;
>
> Thread 2
> x = 2;
> r2 = x;
>
> Let's say I want to justify r1 = r2 = 2.
>
> Clearly I can put x = 1 and x = 2 in C1.
>
> I can clearly add r2 = x with W2(r2=x) = (x=2) in C2, since
> the write x = 2 happens-before the read into r2.
>
> But I think I can only add r1 = x to C3 with W3(r1=x) = (x=1) .
> This is very counterintuitive, but OK, since I can still have
> W(r1=x) = (x=2).
>
> If I'm interpreting this correctly, there at least needs to be
> a footnote explaining the significance of the "i-1" in rule 5.
>
> Hans
>
>
You do seem to be interpreting it correctly.
The point is that you first have to justify a read r occurring
in an execution E_i in which it sees a write that happens before it.
But then, in both E and E_j, j > i, r is allowed to see a different
write.
As you describe above.
So it seems that you did deduce that you could justify a sequentially
consistent
execution, but you found the way it works counterintuitive. Is that
correct?
We'll try to explain it better.
By the way, you could have committed both reads in E2, since in both E2
and E
they see writes that were committed in E1.
Bill
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