D.3 Priority Ceiling Locking
This subclause 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
Post-Compilation Rules
A Locking_Policy
pragma is a configuration pragma.
Dynamic Semantics
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.
The
expression
specified for the Priority or Interrupt_Priority aspect (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
aspect is specified for a protected object, the initial priority is specified
by the locking policy.
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 aspect 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.
The initial ceiling priority of a protected object
is equal to the initial priority for that object.
If an Interrupt_Handler or Attach_Handler aspect
(see
C.3.1) is specified for a protected
subprogram of a protected type that does not have either the Priority
or Interrupt_Priority aspect specified, the initial priority of protected
objects of that type is implementation defined, but in the range of the
subtype System.Interrupt_Priority.
If neither aspect Priority nor Interrupt_Priority
is specified for a protected type, and no protected subprogram of the
type has aspect Interrupt_Handler or Attach_Handler specified, 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.
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
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.
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;
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
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.
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.
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 Advice
The implementation should use names that end with
“_Locking” 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).
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