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D.3 Priority Ceiling Locking

1
[This clause specifies the interactions between priority task scheduling and protected object ceilings. This interaction is based on the concept of the ceiling priority of a protected object.]

Syntax

2
The form of a pragma Locking_Policy is as follows: 
3
  pragma Locking_Policy(policy_identifier);

Legality Rules

4
The policy_identifier shall either be Ceiling_Locking or an implementation-defined identifier.
4.a
Implementation defined: Implementation-defined policy_identifiers allowed in a pragma Locking_Policy.

Post-Compilation Rules

5
{configuration pragma (Locking_Policy) [partial]} {pragma, configuration (Locking_Policy) [partial]} A Locking_Policy pragma is a configuration pragma.

Dynamic Semantics

6/2
{8652/0073} {AI95-00091-01} {AI95-00327-01} {locking policy} [A locking policy specifies the details of protected object locking. All protected objects have a priority. The locking policy specifies the meaning of the priority of a These rules specify whether or not protected object objects have priorities, and the relationships between these priorities and task priorities. In addition, the policy specifies the state of a task when it executes a protected action, and how its active priority is affected by the locking.] The locking policy is specified by a Locking_Policy pragma. For implementation-defined locking policies, the meaning of the priority of effect of a Priority or Interrupt_Priority pragma on a protected object is implementation defined. If no Locking_Policy pragma applies to appears in any of the program units comprising a partition, the locking policy for that partition, as well as the meaning of the priority of effect of specifying either a Priority or Interrupt_Priority pragma for a protected object, are implementation defined. {Priority (of a protected object)}
6.a/2
Implementation defined: The locking policy if no Locking_Policy pragma applies to any unit of a partition.
6.1/2
  {AI95-00327-01} The expression of a Priority or Interrupt_Priority pragma (see D.1) is evaluated as part of the creation of the corresponding protected object and converted to the subtype System.Any_Priority or System.Interrupt_Priority, respectively. The value of the expression is the initial priority of the corresponding protected object. If no Priority or Interrupt_Priority pragma applies to a protected object, the initial priority is specified by the locking policy. {implicit subtype conversion (pragma Priority) [partial]} {implicit subtype conversion (pragma Interrupt_Priority) [partial]}
7
There is one predefined locking policy, Ceiling_Locking; this policy is defined as follows: 
8/2
9/2
10/2
10.a
Implementation defined: Default ceiling priorities.
11/2
12
13

Bounded (Run-Time) Errors

13.1/2
   {AI95-00327-01} Following any change of priority, it is a bounded error for the active priority of any task with a call queued on an entry of a protected object to be higher than the ceiling priority of the protected object. {bounded error (cause) [partial]} In this case one of the following applies:
13.2/2
13.3/2
13.4/2
13.5/2
13.a.1/2
Ramification: Note that the error is “blamed” on the task that did the entry call, not the task that changed the priority of the task or protected object. This seems to make sense for the case of changing the priority of a task blocked on a call, since if the Set_Priority had happened a little bit sooner, before the task queued a call, the entry-calling task would get the error. Similarly, there is no reason not to raise the priority of a task that is executing in an abortable_part, so long as its priority is lowered before it gets to the end and needs to cancel the call. The priority might need to be lowered to allow it to remove the call from the entry queue, in order to avoid violating the ceiling. This seems relatively harmless, since there is an error, and the task is about to start raising an exception anyway. 

Implementation Permissions

14
The implementation is allowed to round all ceilings in a certain subrange of System.Priority or System.Interrupt_Priority up to the top of that subrange, uniformly. 
14.a
Discussion: For example, an implementation might use Priority'Last for all ceilings in Priority, and Interrupt_Priority'Last for all ceilings in Interrupt_Priority. This would be equivalent to having two ceiling priorities for protected objects, “nonpreemptible” and “noninterruptible”, and is an allowed behavior.
14.b
Note that the implementation cannot choose a subrange that crosses the boundary between normal and interrupt priorities.
15/2
 {AI95-00256-01} Implementations are allowed to define other locking policies, but need not support more than one locking such policy per partition.
16
[Since implementations are allowed to place restrictions on code that runs at an interrupt-level active priority (see C.3.1 and D.2.1), the implementation may implement a language feature in terms of a protected object with an implementation-defined ceiling, but the ceiling shall be no less than Priority'Last.] 
16.a
Implementation defined: The ceiling of any protected object used internally by the implementation.
16.b
Proof: This permission follows from the fact that the implementation can place restrictions on interrupt handlers and on any other code that runs at an interrupt-level active priority.
16.c
The implementation might protect a storage pool with a protected object whose ceiling is Priority'Last, which would cause allocators to fail when evaluated at interrupt priority. Note that the ceiling of such an object has to be at least Priority'Last, since there is no permission for allocators to fail when evaluated at a non-interrupt priority. 

Implementation Advice

17
The implementation should use names that end with “_Locking” for implementation-defined locking policies.
17.a/2
Implementation Advice: Names that end with “_Locking” should be used for implementation-defined locking policies.
NOTES
18
23  While a task executes in a protected action, it can be preempted only by tasks whose active priorities are higher than the ceiling priority of the protected object.
19
24  If a protected object has a ceiling priority in the range of Interrupt_Priority, certain interrupts are blocked while protected actions of that object execute. In the extreme, if the ceiling is Interrupt_Priority'Last, all blockable interrupts are blocked during that time.
20
25  The ceiling priority of a protected object has to be in the Interrupt_Priority range if one of its procedures is to be used as an interrupt handler (see C.3).
21
26  When specifying the ceiling of a protected object, one should choose a value that is at least as high as the highest active priority at which tasks can be executing when they call protected operations of that object. In determining this value the following factors, which can affect active priority, should be considered: the effect of Set_Priority, nested protected operations, entry calls, task activation, and other implementation-defined factors.
22
27  Attaching a protected procedure whose ceiling is below the interrupt hardware priority to an interrupt causes the execution of the program to be erroneous (see C.3.1).
23
28  On a single processor implementation, the ceiling priority rules guarantee that there is no possibility of deadlock involving only protected subprograms (excluding the case where a protected operation calls another protected operation on the same protected object).

Extensions to Ada 95

23.a/2
{AI95-00327-01} {extensions to Ada 95} All protected objects now have a priority, which is the value of the Priority attribute of D.5.2. How this value is interpreted depends on the locking policy; for instance, the ceiling priority is derived from this value when the locking policy is Ceiling_Locking.

Wording Changes from Ada 95

23.b/2
{8652/0073} {AI95-00091-01} Corrigendum: Corrected the wording to reflect that pragma Locking_Policy cannot be inside of a program unit.
23.c/2
{AI95-00256-01} Clarified that an implementation need support only one locking policy (of any kind, language-defined or otherwise) per partition.
23.d/2
{AI95-00327-01} The bounded error for the priority of a task being higher than the ceiling of an object it is currently in was moved here from D.5, so that it applies no matter how the situation arises. 

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