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, 3 insertions, 0 deletions

diff --git a/testsuites/tmtests/tm04/task1.c b/testsuites/tmtests/tm04/task1.c
index 9701e739d9..7be2afd7c1 100644
--- a/testsuites/tmtests/tm04/task1.c
+++ b/testsuites/tmtests/tm04/task1.c
@@ -343,6 +343,7 @@ rtems_task Low_tasks(
 {
   rtems_id          id;
   rtems_status_code status;
+  rtems_mode        prev;
 
   task_count--;
 
@@ -379,6 +380,8 @@ rtems_task Low_tasks(
     RTEMS_DEFAULT_OPTIONS,
     RTEMS_NO_TIMEOUT
   );
+
+  rtems_task_mode(RTEMS_PREEMPT, RTEMS_PREEMPT_MASK, &prev);
 }
 
 rtems_task Restart_task(