Date: Tue, 1 Apr 2003 14:53:09 +0200
From: Jesper Larsson Traeff <traff@XXXXXXXXXXXX>
To: mpi-21@XXXXXXXXXXXXX
Subject: Correction to One-sided communications, Section 6.7: Semantics and Correctness
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Dear MPI-2 Group,

the list has been silent for quite some time. Here's an error (I think) and
an issue concerning semantics of one-sided communications, that might 
reactivate discussions.

MPI-2, Section 6.7: Semantics and Correctness of one-sided communications

The rules on p.138 explain when operations are to complete at origins and
targets, and in particular when updates become visible in public and private
copies of windows.

To my understanding:
Rule 5 should state that an update in a location in a private window copy 
of a process becomes visible in the public window when that process perform
a synchronization operation to end the current exposure epoch, that is an
MPI_WIN_FENCE, MPI_WIN_UNLOCK or MPI_WIN_WAIT (and *not*, as written, 
MPI_WIN_POST)

Rule 6 should state that an update to a public window becomes visible in
the private window of the process owning the locations when the window owner
opens the next exposure epoch, that is after the next MPI_WIN_FENCE, 
MPI_WIN_LOCK or MPI_WIN_POST (and *not*, as written, MPI_WIN_WAIT)

Thus, it seems that in rules 5 and 6 the roles of MPI_WIN_POST and MPI_WIN_WAIT
where falsely swapped. Is that so?

The issue here is quite subtle and maybe some "advice to users" should be added.
The MPI-2 standard apparently only guarantees eg. cache coherence (on a window)
after the opening the next access epoch (Rule 6). Barrier synchronization or
explicit synchronization with send/recv's alone does not guarantee this! This 
can likely confuse users who end, say, a phase in a application using MPI-lock
synchronization with an explicit MPI_Barrier, and after the barrier expect
all data now to be visible in the private window copies. In some MPI 
implementations or for some architectures that may not be so!

Regards

Jesper