Priority Ceiling Locking
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

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# D.3 Priority Ceiling Locking

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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

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

#### Legality Rules

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The policy_identifier shall either be Ceiling_Locking or an implementation-defined identifier.

#### Post-Compilation Rules

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A Locking_Policy pragma is a configuration pragma.

#### Dynamic Semantics

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A locking policy specifies the details of protected object locking. These rules specify whether or not protected 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 effect of a Priority or Interrupt_Priority pragma on a protected object is implementation defined. If no Locking_Policy pragma applies to any of the program units comprising a partition, the locking policy for that partition, as well as the effect of specifying either a Priority or Interrupt_Priority pragma for a protected object, are implementation defined.
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There is one predefined locking policy, Ceiling_Locking; this policy is defined as follows:
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- Every protected object has a ceiling priority, which is determined by either a Priority or Interrupt_Priority pragma as defined in D.1. The ceiling priority of a protected object (or ceiling, for short) is an upper bound on the active priority a task can have when it calls protected operations of that protected object.

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- The expression of a Priority or Interrupt_Priority pragma 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 ceiling priority of the corresponding protected object.

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- If an Interrupt_Handler or Attach_Handler pragma (see C.3.1) appears in a protected_definition without an Interrupt_Priority pragma, the ceiling priority of protected objects of that type is implementation defined, but in the range of the subtype System.Interrupt_Priority.

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- If no pragma Priority, Interrupt_Priority, Interrupt_Handler, or Attach_Handler is specified in the protected_definition, then the ceiling priority of the corresponding protected object is System.Priority'Last.

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- While a task executes a protected action, it inherits the ceiling priority of the corresponding protected object.

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- When a task calls a protected operation, a check is made that its active priority is not higher than the ceiling of the corresponding protected object; Program_Error is raised if this check fails.

#### Implementation Permissions

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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.
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Implementations are allowed to define other locking policies, but need not support more than one such policy per partition.
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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.

#### Implementation Advice

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The implementation should use names that end with ``_Locking'' for implementation-defined locking policies.
NOTES
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16 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.
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17 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.
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18 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).
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19 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.
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20 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).
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21 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).

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