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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: is there a version of unix written in Ada
Date: Thu, 1 Oct 2020 17:10:10 -0500
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:rl4thi$hdq$1@gioia.aioe.org...
...
>>> If so, then let there be protected procedure P1 of the object O1 and P2
>>> of O2. If P1 and P2 call to P3 of O3 that would be a problem. Ergo ether
>>> wandering or concurrent protected protected calls must be illegal.
>> But it's not the case...
>
> The scenario is: Task1 calls P1 on O1. Task2 calls P2 on O2. Both P1 and 
> P2 call Protected_Trace on the protected Tracer object. If tasks occupy 
> two different cores, would/should one suspend (in order not use the 
> reserved word "block") another?

A task has to wait to get access to a PO. This is *not* blocking, it is not 
allowed to do anything else during such a period. (This is why protected 
operations are supposed to be fast!). It's canonically implemented with a 
spin-lock, but in some cases one can use lock-free algorithms instead.

For a single core, one can use ceiling locking instead (and have no 
waiting), but that model seems almost irrelevant on modern machines.

                            Randy.