smp: Support ASR path in rtems_task_mode()

cpukit/rtems/src/taskmode.c

@@ -32,11 +32,25 @@ static void _RTEMS_Tasks_Dispatch_if_necessary(
   if ( _Thread_Dispatch_is_enabled() ) {
     bool dispatch_necessary = needs_asr_dispatching;
 
+    /*
+     * FIXME: This locking approach is brittle.  It only works since the
+     * current simple SMP scheduler has no support for the non-preempt mode.
+     */
+#if defined( RTEMS_SMP )
+    ISR_Level level;
+
+    _ISR_Disable( level );
+#endif
+
     if ( !_Thread_Is_heir( executing ) && executing->is_preemptible ) {
       dispatch_necessary = true;
       _Thread_Dispatch_necessary = dispatch_necessary;
     }
 
+#if defined( RTEMS_SMP )
+    _ISR_Enable( level );
+#endif
+
     if ( dispatch_necessary ) {
       _Thread_Dispatch();
     }