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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Protected Objects and Interrupt Handlers
Date: Wed, 24 Feb 2016 18:29:23 +0000
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Date: 2016-02-24T18:29:23+00:00
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Jere <jere.groups@gmail.com> writes:

> I would expect a protected entry or procedure to employ a different
> type of synchronization construct (maybe similar to a
> mutex/semiphore/etc.??) to keep multiple tasks from changing the data
> in the protected object at the same time than is needed for
> interrupts.  For interrupts I would expect something more like
> disabling interrupts to be appropriate. Does a protected
> object/procedure behave differently for interrupts than it does for
> tasks under the hood and Ada is able to handle all those details based
> on the existence of the pragma used?

GNAT (don't forget there are other Ada compilers!) handles differences
between tasks & interrupts calling protected procedures very similarly;
the difference is, I believe, in the way locks are handled.

I don't know too much about the way AdaCore's bare board implementation
does this; the way I dealt with this at [1] was to implement
System.Tasking.Protected_Objects.Lock and .Lock_Read_Only as

   if in an ISR then
      null;
   elsif this PO's ceiling priority is an interrupt priority then
      --  we are in a task, but this PO deals with interrupts, so we
      --  mustn't be interrupted
      disable interrupts;
   else
      --  we are in a task and this PO doesn't deal with interrupts
      take the PO's mutex;
      raise the priority to the PO's ceiling
   end if;

[1] https://sourceforge.net/projects/cortex-gnat-rts/