score: PR2181: Add _Thread_Yield()

The _Scheduler_Yield() was called by the executing thread with thread
dispatching disabled and interrupts enabled.  The rtems_task_suspend()
is explicitly allowed in ISRs:

http://rtems.org/onlinedocs/doc-current/share/rtems/html/c_user/Interrupt-Manager-Directives-Allowed-from-an-ISR.html#Interrupt-Manager-Directives-Allowed-from-an-ISR

Unlike the other scheduler operations the locking was performed inside
the operation.  This lead to the following race condition.  Suppose a
ISR suspends the executing thread right before the yield scheduler
operation.  Now the executing thread is not longer in the set of ready
threads.  The typical scheduler operations did not check the thread
state and will now extract the thread again and enqueue it.  This
corrupted data structures.

Add _Thread_Yield() and do the scheduler yield operation with interrupts
disabled.  This has a negligible effect on the interrupt latency.

1 files changed, 21 insertions, 0 deletions

diff --git a/cpukit/score/include/rtems/score/schedulersmpimpl.h b/cpukit/score/include/rtems/score/schedulersmpimpl.h
index b4126b5d6d..9d74434024 100644
--- a/cpukit/score/include/rtems/score/schedulersmpimpl.h
+++ b/cpukit/score/include/rtems/score/schedulersmpimpl.h
@@ -672,6 +672,27 @@ static inline void _Scheduler_SMP_Change_priority(
   }
 }
 
+static inline void _Scheduler_SMP_Yield(
+  Scheduler_Context     *context,
+  Thread_Control        *thread,
+  Scheduler_SMP_Extract  extract_from_ready,
+  Scheduler_SMP_Enqueue  enqueue_fifo,
+  Scheduler_SMP_Enqueue  enqueue_scheduled_fifo
+)
+{
+  Scheduler_SMP_Node *node = _Scheduler_SMP_Node_get( thread );
+
+  if ( node->state == SCHEDULER_SMP_NODE_SCHEDULED ) {
+    _Scheduler_SMP_Extract_from_scheduled( thread );
+
+    ( *enqueue_scheduled_fifo )( context, thread );
+  } else {
+    ( *extract_from_ready )( context, thread );
+
+    ( *enqueue_fifo )( context, thread );
+  }
+}
+
 static inline void _Scheduler_SMP_Insert_scheduled_lifo(
   Scheduler_Context *context,
   Thread_Control    *thread