score: Simplify _Thread_Change_priority()

The function to change a thread priority was too complex.  Simplify it
with a new scheduler operation.  This increases the average case
performance due to the simplified logic.  The interrupt disabled
critical section is a bit prolonged since now the extract, update and
enqueue steps are executed atomically.  This should however not impact
the worst-case interrupt latency since at least for the Deterministic
Priority Scheduler this sequence can be carried out with a wee bit of
instructions and no loops.

Add _Scheduler_Change_priority() to replace the sequence of
  - _Thread_Set_transient(),
  - _Scheduler_Extract(),
  - _Scheduler_Enqueue(), and
  - _Scheduler_Enqueue_first().

Delete STATES_TRANSIENT, _States_Is_transient() and
_Thread_Set_transient() since this state is now superfluous.

With this change it is possible to get rid of the
SCHEDULER_SMP_NODE_IN_THE_AIR state.  This considerably simplifies the
implementation of the new SMP locking protocols.

1 files changed, 4 insertions, 1 deletions

diff --git a/cpukit/score/src/schedulersimpleyield.c b/cpukit/score/src/schedulersimpleyield.c
index ea41892ad0..65578d0bbb 100644
--- a/cpukit/score/src/schedulersimpleyield.c
+++ b/cpukit/score/src/schedulersimpleyield.c
@@ -26,11 +26,14 @@ void _Scheduler_simple_Yield(
   Thread_Control          *the_thread
 )
 {
+  Scheduler_simple_Context *context =
+    _Scheduler_simple_Get_context( scheduler );
   ISR_Level       level;
 
   _ISR_Disable( level );
 
-    _Scheduler_simple_Ready_queue_requeue( scheduler, the_thread );
+    _Chain_Extract_unprotected( &the_thread->Object.Node );
+    _Scheduler_simple_Insert_priority_fifo( &context->Ready, the_thread );
 
     _ISR_Flash( level );