D.4 Entry Queuing Policies
This clause specifies a mechanism
for a user to choose an entry queuing policy
. It also defines
two such policies. Other policies are implementation defined.
The form of
is as follows:
shall be either FIFO_Queuing, Priority_Queuing or an implementation-defined
pragma is a configuration pragma.
A queuing policy
the order in which tasks are queued for entry service, and the order
in which different entry queues are considered for service. The queuing
policy is specified by a Queuing_Policy pragma.
Two queuing policies, FIFO_Queuing and Priority_Queuing,
are language defined. If no Queuing_Policy pragma applies to any of the
program units comprising the partition, the queuing policy for that partition
is FIFO_Queuing. The rules for this policy are specified in 9.5.3
policy is defined as follows:
- The calls to an
entry (including a member of an entry family) are queued in an order
consistent with the priorities of the calls. The priority of an entry
call is initialized from the active priority of the calling task
at the time the call is made, but can change later. Within the same priority,
the order is consistent with the calling (or requeuing, or priority setting)
time (that is, a FIFO order).
- After a call is first queued, changes
to the active priority of a task do not affect the priority of the call,
unless the base priority of the task is set while the task is blocked
on an entry call.
- When the base priority of a task is
set (see D.5), if the task is blocked on an
entry call, and the call is queued, the priority of the call is updated
to the new active priority of the calling task. This causes the call
to be removed from and then reinserted in the queue at the new active
- When more than one condition
of an entry_barrier
of a protected object becomes True, and more than one of the respective
queues is nonempty, the call with the highest priority is selected. If
more than one such call has the same priority, the call that is queued
on the entry whose declaration is first in textual order in the protected_definition
is selected. For members of the same entry family, the one with the lower
family index is selected.
- When more than one alternative of
is open and has queued calls, an alternative whose queue has the highest-priority
call at its head is selected. If two or more open alternatives have equal-priority
queued calls, then a call on the entry in the accept_alternative
that is first in textual order in the selective_accept
Implementations are allowed to define other queuing
policies, but need not support more than one queuing policy per partition.
Implementations are allowed to defer the reordering
of entry queues following a change of base priority of a task blocked
on the entry call if it is not practical to reorder the queue immediately.
The implementation should use names that end with
“_Queuing” for implementation-defined queuing policies.