>> But broadcasting is a form of message-passing too!
> 
> That wasn't my point.  My point was that there is no return channel.
> If you want to know when your write is done, you have to use a lock
> or a memory barrier.  Both are very expensive.

As I said, it's a matter of abstraction level. My point is:
you can model both abstraction levels with message passing.

Moreover, using shared memory doesn't magically make these
synchronisation issues go away. They remain.

Martin