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Date: Tue, 11 Mar 2008 00:30:01 +0200
From: Niklas Holsti <niklas.holsti@tidorum.invalid>
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Newsgroups: comp.lang.ada
Subject: Re: Robert Dewar's great article about the Strengths of Ada over
 other langauges in multiprocessing!
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Date: 2008-03-11T00:30:01+02:00
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Randy Brukardt wrote:
> "Vadim Godunko" <vgodunko@gmail.com> wrote in message
> news:ec684efe-61a6-4463-bd43-fb5895e868bc@x30g2000hsd.googlegroups.com...
>>
>>I have attach all source code. Protected object used for atomic
>>reference counting...
> 
> If you need an atomic component, why didn't you just declare it as such and
> let the compiler handle the mess? (Yes, an implementation is allowed to
> reject pragma Atomic if it can't handle such, but that should always be OK
> for an integer counter.) A protected object is going to be much heavier than
> Atomic (which just uses the hardware support directly).
> 
> ...
>     type Private_Data is record
>        --Counter : aliased League.Internals.Atomics.Counter;
>        Counter : Natural := 0;
>        pragma Atomic (Counter);

That won't work if the Counter is used for reference counts. 
Vadim's code has parts like

          Self.Value := Self.Value - 1;
          Zero := Self.Value = 0;

and pragma Atomic will not ensure that the whole sequence of 
statements and several uses of Self.Value are a single critical 
(uninterrupted) section. The pragma only means that the whole of 
Self.Value can be read or written atomically, even if it contains 
more than one storage unit. It does not ensure that Self.Value can 
be updated atomically. The sequence "read; modify; write" can be 
interrupted even with pragma Atomic.

As Randy says in another message, pragma Atomic is good foor 
lock-free algorithms (if the memory order semantics are good) but 
it is not enough for lock-based algorithms such as Vadim is using.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .