Re: Ada Protected Object Tutorial #1

[kaz@ashi.footprints.net (Kaz Kylheku)]

On Fri, 17 Dec 1999 00:41:27 GMT, Robert A Duff <bobduff@world.std.com> wrote:
>"James S. Rogers" <jimmaureenrogers@worldnet.att.net> writes:
>
>> This is the first of hopefully several examples of uses for Ada Protected
>> Objects.
>
>People seem to get confused about the difference between waiting for a
>lock to become free, and waiting for a barrier to become true.  Only the
>latter is called "blocking" in Ada 95 terms, and the former is sort of
>swept under the rug.  If your tutorial can explain all that clearly and
>simply, you will have done a great service.

Waiting for locks is inherently different from waiting for conditions to become
true. In the abstract sense, they are both similar: a thread has to be delayed.
But in practical terms, locks are held briefly, so waiting on a lock can be done
without using heavy weight scheduling; e.g. using spinlocks.  Or, in a kernel
running on the bare hardware, a combination of disabling interrupts and
spinning (multiple processors are present). In other words, techniques that
would be disastrous if applied to long waits.