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, 9 insertions, 6 deletions

diff --git a/cpukit/score/include/rtems/score/schedulerimpl.h b/cpukit/score/include/rtems/score/schedulerimpl.h
index 31ae6d184d..7bf4038b5e 100644
--- a/cpukit/score/include/rtems/score/schedulerimpl.h
+++ b/cpukit/score/include/rtems/score/schedulerimpl.h
@@ -314,6 +314,9 @@ RTEMS_INLINE_ROUTINE void _Scheduler_Unblock( Thread_Control *the_thread )
  * must ensure that the priority value actually changed and is not equal to the
  * current priority value.
  *
+ * The operation must update the heir and thread dispatch necessary variables
+ * in case the set of scheduled threads changes.
+ *
  * @param[in] the_thread The thread changing its priority.
  * @param[in] new_priority The new thread priority.
  * @param[in] prepend_it In case this is true, then enqueue the thread as the
@@ -630,16 +633,16 @@ bool _Scheduler_Set_affinity(
 #endif /* defined(__RTEMS_HAVE_SYS_CPUSET_H__) */
 
 RTEMS_INLINE_ROUTINE void _Scheduler_Update_heir(
-  Thread_Control *heir,
-  bool force_dispatch
+  Thread_Control *new_heir,
+  bool            force_dispatch
 )
 {
-  Thread_Control *executing = _Thread_Executing;
-
-  _Thread_Heir = heir;
+  Thread_Control *heir = _Thread_Heir;
 
-  if ( executing != heir && ( force_dispatch || executing->is_preemptible ) )
+  if ( heir != new_heir && ( heir->is_preemptible || force_dispatch ) ) {
+    _Thread_Heir = new_heir;
     _Thread_Dispatch_necessary = true;
+  }
 }
 
 RTEMS_INLINE_ROUTINE void _Scheduler_Generic_block(