[Mpi3-rma] Memory barriers in MPI_WIN_LOCK_ALL mode

[Underwood, Keith D]

> > Ok, but, the kind of thing that delays that visibility (e.g. the
> > uncertainty between when a posted write starts over PCIExpress and
> > when it is known to be globally visible) is completely unaffected by a
> > memory barrier.  Memory barrier until your heart is content and that
> > thing could still be getting retried across a PCIExpress link and the
> > process (and the NIC and other side of the link and just about
> > anything else) would have no way to know...
> 
> Hmm.  Yes, you are right.  That's even worse though :-(.

Not usually.  If you start synchronizing outside the network you better know exactly what you are doing.  Most networks have some ordering primitives to help here.