Re: is there a version of unix written in Ada

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 2020-10-01 19:06, Dmitry A. Kazakov wrote:
> On 01/10/2020 17:38, Niklas Holsti wrote:
> 
>> If the protected procedures belong to different protected objects, yes 
>> it is legal. But not if they belong to the same object, as J-P noted.
> 
> But then you have a problem when two independently running protected 
> procedures of *different* objects call a procedure of a third object. 
> You must serialize these calls, and that is effectively blocking.


I don't know what you mean by "effectively", here, but yes, one of the 
tasks must wait for the other task to complete the protected operation 
on the third object. So what? This is the idea of a protected object: if 
two tasks want to operate on the same object at the same time, the tasks 
must be serialized, and one task must wait for the other. This applies 
even if the task that has to wait is already in the middle of another 
protected operation on another object.

This is analogous to the fact that any task may have to wait for 
higher-priority (possibly pre-empting) tasks to finish their current 
work, and in fact the ceiling-priority-locking method uses this fact to 
implement the protection of protected objects. Even a task that is in 
the middle of a protected operation can be pre-empted by a 
higher-priority task and so may have to wait, as is normal in priority 
scheduling.

While the usual advice is that protected operations should be "short and 
quick", this is as misleading as claiming that a real-time program must 
react "quickly" to inputs. In a given real-time program, the actual 
upper limit on the duration of a protected operation is set only by the 
effect of that operation's execution on the overall schedulability of 
that program. At low priorities, where deadlines usually are relatively 
long, it is quite ok to have protected operations that take quite a 
while to execute, as long as the overall response times of the affected 
low-priority tasks remains shorter than their deadlines. Of course the 
same principle applies at all priority levels.