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From: "John B. Matthews" <nospam@nospam.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Barrier re-evaluation issue with GNAT 4.3.2
Date: Mon, 28 Sep 2009 08:39:28 -0400
Organization: The Wasteland
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Date: 2009-09-28T08:39:28-04:00
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In article <h9q34o$odd$1@news.eternal-september.org>,
 Reto Buerki <reet@codelabs.ch> wrote:

> John B. Matthews wrote:
> > I get the same result with FSF GNAT 4.3.4. I revised your code to follow 
> > the "protected Event" example seen here:
> > 
> > <http://www.adaic.com/standards/95rat/RAThtml/rat95-p1-2.html#9>
> > 
> > It seems to work. The compiler warns, "potentially blocking operation in 
> > protected operation" in Wakeup, although the Signal barrier is always 
> > true. I'm not sure the extra entries _should_ be required, but I think 
> > it might be more reliable in the face of multiple threads calling Wait. 
> > I don't know a reason why it wouldn't work under 4.3.2.
> 
> Thanks for your effort in confirming the issue with GNAT 4.3.4 and for
> the "protected Event" example.

You're welcome.

> We will see how we can integrate your idea into our project as a
> workaround for the re-evaluation problem.

This alternative approach also works, without using requeue:

<http://www.adaic.com/standards/95rat/RAThtml/rat95-p2-9.html#2>

I am troubled by the "potentially blocking operation" warning. Two 
things make me think it may be irrelevant: 1) Signal does not actually 
block, except to dequeue waiters, and 2) Wakeup, which enters Signal, is 
itself a protected procedure:

<http://www.adaic.com/standards/05rm/html/RM-D-15.html>

I'd welcome critical thoughts.

-- 
John B. Matthews
trashgod at gmail dot com
<http://sites.google.com/site/drjohnbmatthews>