score: Update _Thread_Heir only if necessary

Previously, the _Thread_Heir was updated unconditionally in case a new
heir was determined.  The _Thread_Dispatch_necessary was only updated in
case the executing thread was preemptible or an internal thread was
unblocked.  Change this to update the _Thread_Heir and
_Thread_Dispatch_necessary only in case the currently selected heir
thread is preemptible or a dispatch is forced.  Move the schedule
decision into the change priority operation and use the schedule
operation only in rtems_task_mode() in case preemption is enabled or an
ASR dispatch is necessary.  This is a behaviour change.  Previously, the
RTEMS_NO_PREEMPT also prevented signal delivery in certain cases (not
always).  Now, signal delivery is no longer influenced by
RTEMS_NO_PREEMPT.  Since the currently selected heir thread is used to
determine if a new heir is chosen, non-preemptible heir threads
currently not executing now prevent a new heir.  This may have an
application impact, see change test tm04.  Document this change in sp04.

Update #2273.

1 files changed, 2 insertions, 0 deletions

diff --git a/cpukit/score/src/schedulerprioritychangepriority.c b/cpukit/score/src/schedulerprioritychangepriority.c
index 06c5f0f7c6..f883e02d43 100644
--- a/cpukit/score/src/schedulerprioritychangepriority.c
+++ b/cpukit/score/src/schedulerprioritychangepriority.c
@@ -59,5 +59,7 @@ Scheduler_Void_or_thread _Scheduler_priority_Change_priority(
     );
   }
 
+  _Scheduler_priority_Schedule_body( scheduler, the_thread, false );
+
   SCHEDULER_RETURN_VOID_OR_NULL;
 }