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D.2.4 Non-Preemptive Dispatching

{AI95-00298-01} {AI05-0299-1} [This subclause clause defines a non-preemptive task dispatching policy.] 

Static Semantics

{AI95-00298-01} {AI95-00355-01} The policy_identifier Non_Preemptive_FIFO_Within_Priorities is a task dispatching policy.
  {AI05-0166-1} The following language-defined library package exists: 
package Ada.Dispatching.Non_Preemptive is
  pragma Preelaborate(Non_Preemptive);
  procedure Yield_To_Higher;
  procedure Yield_To_Same_Or_Higher renames Yield;
end Ada.Dispatching.Non_Preemptive;
  {AI05-0166-1} {AI05-0264-1} A call of Yield_To_Higher is a task dispatching point for this policy. If the task at the head of the highest priority ready queue has a higher active priority than the calling task, then the calling task is preempted.
Ramification: For language-defined policies other than Non_Preemptive_FIFO_Within_Priorities, a higher priority task should never be on a ready queue while a lower priority task is executed. Thus, for such policies, Yield_To_Higher does nothing.
Yield_To_Higher is not a potentially blocking operation; it can be used during a protected operation. That is allowed, as under the predefined Ceiling_Locking policy any task with a higher priority than the protected operation cannot call the operation (that would violate the locking policy). An implementation-defined locking policy may need to define the semantics of Yield_To_Higher differently.

Legality Rules

{AI95-00355-01} Non_Preemptive_FIFO_Within_Priorities shall not be specified as the policy_identifier of pragma Priority_Specific_Dispatching (see D.2.2).
Reason: The non-preemptive nature of this policy could cause the policies of higher priority tasks to malfunction, missing deadlines and having unlimited priority inversion. That would render the use of such policies impotent and misleading. As such, this policy only makes sense for a complete system. 

Dynamic Semantics

{AI95-00298-01} When Non_Preemptive_FIFO_Within_Priorities is in effect, modifications to the ready queues occur only as follows:
{AI95-00298-01} When a blocked task becomes ready, it is added at the tail of the ready queue for its active priority.
When the active priority of a ready task that is not running changes, or the setting of its base priority takes effect, the task is removed from the ready queue for its old active priority and is added at the tail of the ready queue for its new active priority.
When the setting of the base priority of a running task takes effect, the task is added to the tail of the ready queue for its active priority.
When a task executes a delay_statement that does not result in blocking, it is added to the tail of the ready queue for its active priority. 
Ramification: If the delay does result in blocking, the task moves to the “delay queue”, not to the ready queue. 
{AI05-0166-1} For this policy, blocking or termination of a task, a non-blocking delay_statement, a call to Yield_To_Higher, and a call to Yield_To_Same_Or_Higher or Yield are is the only non-blocking event that is a task dispatching points point (see D.2.1).
Ramification: {AI05-0166-1} A delay_statement is always a task dispatching point even if it is not blocking. Similarly, a call to Yield_To_Higher is never blocking, but it is a task dispatching point In each of these cases, they can cause the current task to stop running (it is still ready). Otherwise, the running task continues to run until it is blocked. 

Implementation Requirements

 {AI95-00333-01} An implementation shall allow, for a single partition, both the task dispatching policy to be specified as Non_Preemptive_FIFO_Within_Priorities and also the locking policy (see D.3) to be specified as Ceiling_Locking. 
Reason: This is the preferred combination of the Non_Preemptive_FIFO_Within_Priorities policy with a locking policy, and we want that combination to be portable.

Implementation Permissions

 {AI95-00298-01} {AI05-0229-1} {AI05-0269-1} Since implementations are allowed to round all ceiling priorities in subrange System.Priority to System.Priority'Last (see D.3), an implementation may allow a task of a partition using the Non_Premptive_FIFO_Within_Priorities policy to execute within a protected object without raising its active priority provided the associated protected unit does not contain any subprograms with aspects Interrupt_Handler or Attach_Handler specified, nor does the unit have aspect pragma Interrupt_Priority  specified. When the locking policy (see D.3) is Ceiling_Locking, an implementation taking advantage of this permission shall ensure that a call to Yield_to_Higher that occurs within a protected action uses the ceiling priority of the protected object (rather than the active priority of the task) when determining whether to preempt the task, Interrupt_Handler, or Attach_Handler.
Reason: {AI05-0269-1} We explicitly require that the ceiling priority be used in calls to Yield_to_Higher in order to prevent a risk of priority inversion and consequent loss of mutual exclusion when Yield_to_Higher is used in a protected object. This requirement might lessen the value of the permission (as the current Ceiling_Priority will have to be maintained in the TCB), but loss of mutual exclusion cannot be tolerated. The primary benefit of the permission (eliminating the need for preemption at the end of a protected action) is still available. As noted above, an implementation-defined locking policy will need to specify the semantics of Yield_to_Higher, including this case. 

Extensions to Ada 95

{AI95-00298-01} {AI95-00355-01} Policy Non_Preemptive_FIFO_Within_Priorities is new. 

Extensions to Ada 2005

{AI05-0166-1} Package Dispatching.Non_Preemptive is new. 

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