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
to represent the task's held
The following language-defined
library package exists:
Preelaborate, Nonblocking, Global => in out synchronized is(Asynchronous_Task_Control);
Hold(T : in
Continue(T : in
Is_Held(T : Ada.Task_Identification.Task_Id)
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.
The held state should not
be confused with the blocked state as defined in 9.2
the task is still ready.
For any priority below System.Any_Priority'First, the task dispatching
policy is FIFO_Within_Priorities.
To be honest: This applies even if a
Task_Dispatching_Policy specifies the policy for all of the priorities
of the partition.
Ramification: A task at the held priority
never runs, so it is not necessary to implement FIFO_Within_Priorities
for systems that have only one policy (such as EDF_Across_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.
Ramification: For example, if T is currently
inheriting priorities from other sources (e.g. it is executing in a protected
action), its active priority does not change, and it continues to execute
until it leaves the protected action.
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.
Discussion: Note that the state of T
can be changed immediately after Is_Held returns.
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.
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.
An implementation may omit need
if it is infeasible to support it in the target environment.
Reason: A direct implementation of the
Asynchronous_Task_Control semantics using priorities is not necessarily
efficient enough. Thus, we envision implementations that use some other
mechanism to set the “held” state. If there is no other such
mechanism, support for Asynchronous_Task_Control might be infeasible,
because an implementation in terms of priority would require one idle
task per processor. On some systems, programs are not supposed to know
how many processors are available, so creating enough idle tasks would
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
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.
Extensions to Ada 95
Asynchronous_Task_Control is now Preelaborated,
so it can be used in preelaborated units.
Wording Changes from Ada 95
The description of held tasks was changed to reflect that the calculation
of active priorities depends on the dispatching policy of the base priority.
Thus, the policy of the held priority was specified in order to avoid
surprises (especially when using the EDF policy).
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe