Re: multiple delay alternative

["J-P. Rosen" <rosen@adalog.fr>]

Le 06/03/2018 à 02:50, Mehdi Saada a écrit :
> Before that, one thing I can't get, even after asking other people: 
> for protected objects, in a protected procedure, when it's blocked,
> because a reading (protected function) or an entry is being served,
> are the calls to the procedure put on a queue as with entries ? 
> Someone said me the advantage of protected procedures was that you
> don't have to wait, but I read also they were exclusive with each
> others... damn.
This is kind of subtle.

From the formal (i.e. lawyer) POV:
Queues are not required by the language. On a multi-processor, one
processor could be actively looping on a lock until the lock is
released. OTOH, if you are using the Ceiling_Locking locking policy on a
mono-processor, the management of priorities is sufficient to guarantee
exclusive access without requiring queues.

The important point is that the time a task is blocked waiting on the
lock can be computed. Hence the ban on "potentially blocking operations"
in protected operations.

From the implementation POV:
Nothing prevents an implementation from using a conventional queue of
tasks waiting for the protection lock.

-- 
J-P. Rosen
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