Re: protected type interrupts

["Adam Beneschan" <adam@irvine.com>]

REH wrote:
> Adam Beneschan wrote:
> > REH wrote:
> >
> > > Thanks for the reference.  Is your work-around valid?  Isn't calling
> > > Relay_Object.Isr a potentially blocking operation, and thus a bad thing
> > > to do in an interrupt?
> >
> > I don't think it's potentially blocking... Relay_Object.Isr is a
> > procedure, and as I read 9.5.1, a call to a protected *procedure* is
> > not a potentially blocking operation unless it's done from the same
> > protected object.  A call on a protected *entry* would be potentially
> > blocking.  [This is from the Ada 95 manual.  If this has changed in Ada
> > 0Y, I can't tell because www.adaic.com isn't responding.]
> >
>
> I don't understand.  How can a procedure within a protected object
> modify data internal to the object in a thread-safe way without the
> potentially blocking effect of enforcing mutual exclusion?  If one task
> is "using" the object, and another calls one of the object's
> procedures, would the second task not be blocked until the first one
> exits the object?

I'm just reading what the RM says.  But, although I'm not an expert at
this, I don't believe your argument makes sense given the way protected
objects are supposed to work.  A task cannot, of course, access data
internal to a protected object directly; it has to call one of the PO's
subprograms or entries to get it.  When it does so, then assuming
Ceiling_Locking is in effect, no other task can use the PO---but it's
not because other tasks are blocked, it's simply because the protected
action runs at a higher priority than any task that could use the PO
(and the protected action is supposed to complete very quickly).  So no
mutual exclusion or blocking is necessary when calling protected
subprograms.  At least I think that's how it works.

                                 -- Adam