RE: JavaMemoryModel: The range of memory models now in play

From: Bill Pugh (
Date: Thu Aug 08 2002 - 20:14:54 EDT

At 4:39 PM -0500 8/8/02, Sarita Adve wrote:
>Right. To eliminate potential confusion in future discussion, I want to
>clarify further:
>Simply saying that an execution trace is consistent with program order
>is also "content-free" if there is no other relationship between this
>total order and the values that are read.

In my terminology/description, the values seen by reads are
determined by the memory model.

>In other words, it's fine to
>say that my model has a totally ordered execution trace that is
>consistent with program order, but this statement doesn't imply
>anything. Also, it doesn't preclude talking about my model in the future
>as having a totally ordered execution trace that is not consistent with
>program order.

Yes, to summarize.

For the purposes of discussion, I describe two memory models, A and B.

The only thing significant about these models are the behaviors they
allow. I believe that all the models being considered allow behaviors
that somewhere between the behaviors allowed by A and the behaviors
allowed by B. Whether these models are specified in a manner similar
to A and B is not significant.

In both models A and B, we consider the legality of an execution
trace E of a program P. An execution trace is a totally ordered set
of actions. The actions of each thread T of E occur in the program
order of P and are consistent with intra-thread semantics of E
(excluding the values of reads, which are determined by the memory
model and not the intra-thread semantics).

There is a hb order on E, induced by unlock/lock and volatile write/read pairs.

In both A and B, the values read in E must be consistent with hb;
i.e., if R is a read that returns the value of a write W, then we
cannot have R hb
W and we cannot have a W' to the same location where W hb W' hb R in E.

In model B only, a read must see the value of a previous write.
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