D.11 Asynchronous Task Control
This subclause introduces a language-defined package
to do asynchronous suspend/resume on tasks. It uses a conceptual held
priority value to represent the task's held state.
Static Semantics
The following language-defined
library package exists:
with Ada.Task_Identification;
package Ada.Asynchronous_Task_Control
with Preelaborate, Nonblocking, Global =>
in out synchronized is
procedure Hold(T :
in Ada.Task_Identification.Task_Id);
procedure Continue(T :
in Ada.Task_Identification.Task_Id);
function Is_Held(T : Ada.Task_Identification.Task_Id)
return Boolean;
end Ada.Asynchronous_Task_Control;
Dynamic Semantics
After
the Hold operation has been applied to a task, the task becomes
held.
For each processor there is a conceptual
idle task, which is always
ready. The base priority of the idle task is below System.Any_Priority'First.
The
held priority is a constant of the type Integer whose value
is below the base priority of the idle task.
For any priority below System.Any_Priority'First,
the task dispatching policy is FIFO_Within_Priorities.
The Hold operation sets the state of T to held. For
a held task, the active priority is reevaluated as if the base priority
of the task were the held priority.
The Continue operation resets the state of T to not-held;
its active priority is then reevaluated as determined by the task dispatching
policy associated with its base priority.
The Is_Held function returns True if and only if
T is in the held state.
As part of these operations, a check is made that
the task identified by T is not terminated. Tasking_Error is raised if
the check fails. Program_Error is raised if the value of T is Null_Task_Id.
Erroneous Execution
If any operation in this package
is called with a parameter T that specifies a task object that no longer
exists, the execution of the program is erroneous.
Implementation Permissions
An implementation may omit support for Asynchronous_Task_Control
if it is infeasible to support it in the target environment.
NOTE 1 It is a consequence of the
priority rules that held tasks cannot be dispatched on any processor
in a partition (unless they are inheriting priorities) since their priorities
are defined to be below the priority of any idle task.
NOTE 2 The effect of calling Get_Priority
and Set_Priority on a Held task is the same as on any other task.
NOTE 3 Calling Hold on a held task
or Continue on a non-held task has no effect.
NOTE 4 The
rules affecting queuing are derived from the above rules, in addition
to the normal priority rules:
When a held task is on the ready
queue, its priority is so low as to never reach the top of the queue
as long as there are other tasks on that queue.
If a task is executing in a protected
action, inside a rendezvous, or is inheriting priorities from other sources
(e.g. when activated), it continues to execute until it is no longer
executing the corresponding construct.
If a task becomes held while waiting
(as a caller) for a rendezvous to complete, the active priority of the
accepting task is not affected.
If a task becomes held while waiting
in a
selective_accept,
and an entry call is issued to one of the open entries, the corresponding
accept_alternative
executes. When the rendezvous completes, the active priority of the accepting
task is lowered to the held priority (unless it is still inheriting from
other sources), and the task does not execute until another Continue.
The same holds if the held task is
the only task on a protected entry queue whose barrier becomes open.
The corresponding entry body executes.
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