Re: D.10

[Robert A Duff <bobduff@world.std.com>]

Matthew Heaney <matthew_heaney@acm.org> writes:

> Is an invocation of operation Suspend_Until_True on a suspension object
> by one task atomic with respect to an invocation of Suspend_Until_True
> on the same suspension object by another task?

Well, the RM doesn't say it is, so I guess it isn't.  I admit the RM
isn't crystal clear on this point.

The *intent* is that this feature would be used in a situation where a
given suspension object is "owned" by a single task, and only that task
does Suspend_Until_True.  That task publishes a handle on the suspension
object to the rest of the world, and other tasks can call Set_True to
wake it up.  I would also expect that only the owner does Set_False
(presumably before "publishing").  Actually, it is set False by default
initialization, so you don't need to ever call Set_False, unless you
want to reuse the same suspension object.

The example you gave from Burns and Wellings seems weird to me,
whatever the RM says.

The run-time system I'm currently working on (for the ADI SHARC DSP
chip) has an implementation (just a few instructions) that will make the
example work.  But it wouldn't surprise me if other implementations have
a race condition.

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.