Semantics of requeue

[Moti Ben-Ari <ntbenari@wis.weizmann.ac.il>]

I have a question on the semantics of requeue:

When a requeue is done on an "internal" entry of a protected object,
it is clear that the caller is queued as part of the
"on-going protected action" (LRM 9.5.4(10)).
This prevents nasty race conditions, as shown in many
published examples of concurrent programs written in Ada 95.

However, I am uncertain exactly what is supposed to happen when
a requeue is done on an entry of another protected object or
on an entry of a task.
Specifically, suppose task T1 executing PO.E requeues on entry T2.E.
Is it possible that between the end of the
protected action PO.E, and the (re-)queueing of the caller T1
on the entry T2.E, another task T3 could execute T2.E and
"jump the queue" (assuming default FIFO scheduling)?

LRM 9.5.4(7-8) says:

7   ... The entry_body or accept_statement enclosing the
requeue_statement is then completed, finalized, and left (see 7.6.1).


8   For the execution of a requeue on an entry of a target task,
after leaving the enclosing callable construct, ...
the requeued call is either selected immediately or queued,
as for a normal entry call (see 9.5.3).

Thus I am asking: are paragraphs 7-8 part of a single atomic
statement, or can the scheduler preempt a task between 7 and 8.

Thanks

Moti Ben-Ari

Dr. Mordechai (Moti) Ben-Ari
Dept. of Science Teaching, Weizmann Institute of Science
Rehovot 76100 Israel, Tel: 972-8-934-2940. Fax: 972-8-934-4174.
ntbenari@wis.weizmann.ac.il