The MPI standard seems to allow access to the local window without requiring a lock (at least, I couldn’t find any text requiring a lock). Does this mean that the following example is correct? P0: Win_lock(P1) Put(X, 1) Win_unlock(P1) MPI_Send(P1) P1: MPI_Recv(P0) assert(X == 1) If the above is correct, shouldn’t there be a memory read barrier on P1 somewhere? Since P1 is not making any RMA calls, I’d assume that’ll need to somehow come from the lock and unlock operations. That is, the MPI implementation will need to do an active message in Win_lock and Win_unlock forcing a memory barrier at the target. Assuming that’s correct, I'll have to send out a lock packet even if the user gave the MPI_MODE_NOCHECK hint, for memory consistency reasons. That sounds awful, so I’m really hoping that I missed something in the standard which will say I don’t need to do all this. Note that all this active message problem will go away if I need P1 to do a lock/unlock to itself in order to access X. Thanks, — Pavan