RE: JavaMemoryModel: JSR 133 Status report

From: David Holmes (
Date: Sun Jul 14 2002 - 12:08:26 EDT

Bill Pugh wrote:
> As you know, we had decided to relax volatile variables so that they
> weren't quite sequentially consistent.

We had? Can you restate (in simple terms) in what way volatiles *are*
sequentially consistent.

> Ex2
> Thread 1:
> x = 1
> Thread 2:
> r1 = z
> r2 = x
> y = r2
> Thread 3:
> r3 = y
> z = r3
> The question is: can this program result in r1=1?
> We need for this result to be possible. A compiler could easily
> decide to reorder the read of z in thread 2 to the last action in
> thread 2, and then an a sequentially consistent execution would allow
> r1=1. Under our previous semantics however, it could not. For that to
> happen, we would need to perform an initWrite of y=1 in thread 2
> before reading z. But at that point, we can't guarantee that a 1 will
> in fact be written to y. We can't use a guaranteed read because
> guaranteed reads are restricted to see properly synchronized reads.

I don't quite see the problem in terms of the semantics of the model. But
given the example uses absolutely no sync, I can't see anything to stop the
reordering within thread 3 to do the read of z last.

> Jeremy and I are focused on this now, and we hope to have JSR ready
> for public review by the end of July.

You mean community review - right?

David Holmes

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