[Mpi3-rma] MPI-3 UNIFIED model clarification

[Jed Brown]

Pavan Balaji <balaji at mcs.anl.gov> writes:
> I guess the question here is -- when can the target process read the 
> data written by the origin process?  In the separate model, the answer 
> is simple -- we do the necessary synchronization and then do a WIN_SYNC 
> to synchronize the two windows.  In the unified model, it's unclear 
> since the user does not need to a WIN_SYNC.

This is lack of clarity in the standard, but I think not lack of clarity
in practice (to a user familiar with memory fences), provided that
certain functions (like MPI_Win_sync) contain suitable memory fences.

I don't think it's worth talking about x86 specifically.  Rather, I
would define semantics in terms of a weak memory model (C11 or
Alpha/Linux kernel).  On architectures with stronger memory models, some
fences become no-ops, but that doesn't have to be discussed in the
standard.
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