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From: AdaMagica <christoph.grein@eurocopter.com>
Newsgroups: comp.lang.ada
Subject: Re: About task-safeness
Date: Thu, 3 Feb 2011 03:22:53 -0800 (PST)
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Date: 2011-02-03T03:22:53-08:00
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On 3 Feb., 12:07, Georg Bauhaus <rm.dash-bauh...@futureapps.de> wrote:
> > Randy, could you be more explicit about your suggested use of Atomic? A=
s I
> > understand it, even if a counter variable N is Atomic, two tasks concur=
rently
> > executing an assignment of the form
>
> > =A0 =A0N :=3D N + 1;
>
> > can interleave their actions so that N is increased by only 1, not by 2=
 as
> > intended.
>
> When I first read about atomic counting, I thought so, too. =A0But then,
> I noticed a new(?) Implementation Advice in the LRM that says, IIRC,
> that compilers should make atomic processor ops available, including
> decrement. =A0A neighboring paragraph is about intrinsic operations.

You mean C.1(11-12):
(11) It is recommended that intrinsic subprograms be provided for
convenient access to any machine
operations that provide special capabilities or efficiency and that
are not otherwise available through the
language constructs. Examples of such instructions include:
(12) =95 Atomic read-modify-write operations =97 e.g., test and set,
compare and swap, decrement and test,
enqueue/dequeue.

But N :=3D N + 1; is not such an operation even if N is atomic. An
implementation would have to provide something like a CAS operation.