Re: Looking for implementation idea

[Corey Minyard <minyard@acm.org>]

stt@houdini.camb.inmet.com (Tucker Taft) writes:
> : Thanks for the reply.  I would expect the average programmer would
> : think of a protected type like a Java one, something more
> : semaphore-like.  I knew that it wasn't (and I know the reasons), but I
> : would expect that when the average programmer puts something in a
> : protected type they would expect it to be "protected" by mutex, which
> : wouldn't happen on an SMP machine, but would on a uniprocessor one.
> 
> I've lost you here.  What "wouldn't happen on an SMP machine?"
> 
> On an SMP operating system, O/S mutexes work across processors.
> 
> : So should I now put spin-locks in all my protected type operations so
> : they will provide mutex on SMP machines (if mutex is what I am looking
> : for, which I expect is their most common use)?
> 
> No, now I must have really confused you.  The *implementation* of
> protected types generally uses spin locks on a multi-processor (though
> other mechanisms are also possible).  The programmer need not worry
> about this at all: mutual exclusion is provided by protected types, even 
> if there are multiple processors.
> 

Thanks.  That clears things up substantually!  I thought that
protected types were a priority-only thing, that they had no mutex
built-in.  They obviously still use priority, but are a more general
purpose mutex.  I guess I should have looked at the RM (9.5.1), it's
pretty clear there.

-- 
Corey Minyard                   Internet:  minyard@acm.org
  Work: minyard@nortelnetworks.com  UUCP:  minyard@wf-rch.cirr.com