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D.3 Priority Ceiling Locking
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
The form of
a pragma Locking_Policy is as follows:
shall either be Ceiling_Locking or an implementation-defined identifier
pragma is a configuration pragma.
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
There is one predefined
locking policy, Ceiling_Locking; this policy is defined as follows:
- 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.
- 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.
- 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
- 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.
- While a task executes a protected
action, it inherits the ceiling priority of the corresponding protected
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.
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.
Implementations are allowed to define other locking
policies, but need not support more than one such policy per partition.
Since implementations are allowed to place restrictions
on code that runs at an interrupt-level active priority (see C.3.1
), 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.
The implementation should use names that end
with ``_Locking'' for implementation-defined locking policies.
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.
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.
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).
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.
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).
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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