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Date: Fri, 1 Feb 2019 18:13:39 -0800 (PST)
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Subject: Re: Ravenscar References
From: Jere <jhb.chat@gmail.com>
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Xref: reader01.eternal-september.org comp.lang.ada:55429
Date: 2019-02-01T18:13:39-08:00
List-Id: <comp.lang.ada>

On Tuesday, January 29, 2019 at 2:24:19 PM UTC-5, Niklas Holsti wrote:
> On 19-01-29 03:08 , Jorge Real wrote:
> > El jueves, 17 de enero de 2019, 6:31:34 (UTC+1), J-P. Rosen  escribi=C3=
=B3:
> >> Le 16/01/2019 =C3=A0 18:48, lyttlec a =C3=A9crit :
> >>> Can anyone suggest a good reference on using the ravenscar profile? I=
n
> >>> the Ada books I have, it only gets a one or two page mention. A
> >>> reference with an extended case study would be great.
> >>>
> >> Did you get the official report at
> >> http://www.open-std.org/jtc1/sc22/wg9/n424.pdf
> >> ?
> >
> > Note there is a revised version of of the Ravenscar guide at
> > http://www.open-std.org/JTC1/SC22/WG9/n575.pdf
> >
> > This version of June 2017 is adapted to Ada 2012 TC1.
>=20
> But, disappointingly, it still contains the (IMO) misleading advice that=
=20
> "All global objects that are either of a protected type or an atomic=20
> type may be safely shared".
>=20
> IMO atomic-type objects can be safely shared only if care is taken to=20
> follow a proper lock-free read/write protocol.
>=20

I got the impression from the entire section there that the author was=20
referring to erroneous access from data races, which would make the
statement "technically" correct.  The X :=3D X + 1 case is not=20
technically erroneous (not a data race).  It's instead considered
a race condition (which is not technically erroneous).  Atomic
variables don't do anything to protect you from race conditions,=20
but they definite make you safe from data races.

That doesn't detract from the meat of your general point that=20
protected objects are much more capable of preventing unwanted
code results than atomic variables and that atomic variables
should be used with care.  I just don't think the author was=20
intending to go into race conditions and instead focused on=20
data races (erroneous access), at least for that section of the
paper.