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 object.]
Syntax
The form of a
pragma
Locking_Policy is as follows:
Legality Rules
Implementation defined: Implementation-defined
policy_identifiers
allowed in a
pragma
Locking_Policy.
Post-Compilation Rules
{configuration pragma
(Locking_Policy) [partial]} {pragma,
configuration (Locking_Policy) [partial]} A
Locking_Policy pragma is a configuration pragma.
Dynamic Semantics
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{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 protected object, 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 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 meaning of the priority of a protected object, are implementation
defined.
{Priority (of a protected object)}
Implementation defined: The locking policy
if no Locking_Policy pragma applies to any unit of a partition.
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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]}
There is one predefined
locking policy, Ceiling_Locking; this policy is defined as follows:
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{ceiling priority (of a protected object)}
Every protected object has a
ceiling priority,
which is determined by either a Priority or Interrupt_Priority pragma
as defined in
D.1, or by assignment to the
Priority attribute as described in
D.5.2.
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 initial ceiling priority of a protected object is equal to the initial
priority for that 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 initial priority of protected
objects of that type is implementation defined, but in the range of the
subtype System.Interrupt_Priority.
Implementation defined: Default ceiling
priorities.
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If no
pragma
Priority, Interrupt_Priority, Interrupt_Handler, or Attach_Handler is
specified in the
protected_definition,
then the initial 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 object.
{Ceiling_Check
[partial]} {check,
language-defined (Ceiling_Check)} {Program_Error
(raised by failure of run-time check)} 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.
Bounded (Run-Time) Errors
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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:
at any time prior to executing the entry body Program_Error
is raised in the calling task;
{Program_Error
(raised by failure of run-time check)} when the entry is open the entry body is executed
at the ceiling priority of the protected object;
when the entry is open the entry body is executed
at the ceiling priority of the protected object and then Program_Error
is raised in the calling task; or
{Program_Error
(raised by failure of run-time check)} when the entry is open the entry body is executed
at the ceiling priority of the protected object that was in effect when
the entry call was queued.
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
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.
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.
Note that the implementation cannot choose a
subrange that crosses the boundary between normal and interrupt priorities.
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Implementations are allowed to define other locking policies, but need
not support more than one locking policy per partition.
[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 defined: The ceiling of
any protected object used internally by the implementation.
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.
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
The implementation should use names that end with
“_Locking” for implementation-defined locking policies.
Implementation Advice: Names that end
with “_Locking” should be used for implementation-defined
locking policies.
20 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.
21 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.
22 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).
23 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.
24 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).
25 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
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{
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
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Corrigendum: Corrected the wording to reflect that pragma Locking_Policy
cannot be inside of a program unit.
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Clarified that an implementation need support only one locking policy
(of any kind, language-defined or otherwise) per partition.
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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.
Sponsored by Ada-Europe