score: Fix scheduler yield in SMP configurations

Check that no ask help request is registered during unblock and yield
scheduler operations.  There is no need to ask for help if a scheduled
thread yields, since this is already covered by the pre-emption
detection.

Update #2556.

cpukit/score/Makefile.am

@@ -161,6 +161,7 @@ libscore_a_SOURCES += src/cpusetprintsupport.c
 libscore_a_SOURCES += src/schedulerdefaultaskforhelp.c
 libscore_a_SOURCES += src/schedulerdefaultgetaffinity.c
 libscore_a_SOURCES += src/schedulerdefaultsetaffinity.c
+libscore_a_SOURCES += src/schedulersmp.c
 libscore_a_SOURCES += src/schedulersmpstartidle.c
 endif
 

cpukit/score/include/rtems/score/schedulerimpl.h

@@ -10,7 +10,7 @@
 /*
  *  Copyright (C) 2010 Gedare Bloom.
  *  Copyright (C) 2011 On-Line Applications Research Corporation (OAR).
- *  Copyright (c) 2014, 2016 embedded brains GmbH
+ *  Copyright (c) 2014, 2017 embedded brains GmbH
  *
  *  The license and distribution terms for this file may be
  *  found in the file LICENSE in this distribution or at
@@ -114,8 +114,10 @@ RTEMS_INLINE_ROUTINE void _Scheduler_Release_critical(
 }
 
 #if defined(RTEMS_SMP)
+void _Scheduler_Request_ask_for_help( Thread_Control *the_thread );
+
 /**
- * @brief Registers an ask for help request.
+ * @brief Registers an ask for help request if necessary.
  *
  * The actual ask for help operation is carried out during
  * _Thread_Do_dispatch() on a processor related to the thread.  This yields a
@@ -130,22 +132,7 @@ RTEMS_INLINE_ROUTINE void _Scheduler_Ask_for_help( Thread_Control *the_thread )
   _Assert( _Thread_State_is_owner( the_thread ) );
 
   if ( the_thread->Scheduler.helping_nodes > 0 ) {
-    ISR_lock_Context  lock_context;
+    _Scheduler_Request_ask_for_help( the_thread );
-    Per_CPU_Control  *cpu;
-
-    _Thread_Scheduler_acquire_critical( the_thread, &lock_context );
-    cpu = _Thread_Get_CPU( the_thread );
-    _Per_CPU_Acquire( cpu );
-
-    _Chain_Append_unprotected(
-      &cpu->Threads_in_need_for_help,
-      &the_thread->Scheduler.Help_node
-    );
-
-    _Per_CPU_Release( cpu );
-    _Thread_Scheduler_release_critical( the_thread, &lock_context );
-
-    _Thread_Dispatch_request( _Per_CPU_Get(), cpu );
   }
 }
 #endif

cpukit/score/include/rtems/score/schedulersmpimpl.h

@@ -1085,8 +1085,8 @@ static inline void _Scheduler_SMP_Yield(
 
   if ( node_state == SCHEDULER_SMP_NODE_SCHEDULED ) {
     _Scheduler_SMP_Extract_from_scheduled( node );
-
+    ( *enqueue_scheduled_fifo )( context, node );
-    needs_help = ( *enqueue_scheduled_fifo )( context, node );
+    needs_help = false;
   } else if ( node_state == SCHEDULER_SMP_NODE_READY ) {
     ( *extract_from_ready )( context, node );
 

cpukit/score/src/schedulersmp.c

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2014, 2017 embedded brains GmbH.
+ *
+ * The license and distribution terms for this file may be
+ * found in the file LICENSE in this distribution or at
+ * http://www.rtems.org/license/LICENSE.
+ */
+
+#if HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include <rtems/score/schedulersmpimpl.h>
+
+void _Scheduler_Request_ask_for_help( Thread_Control *the_thread )
+{
+  ISR_lock_Context lock_context;
+
+  _Thread_Scheduler_acquire_critical( the_thread, &lock_context );
+
+  if ( _Chain_Is_node_off_chain( &the_thread->Scheduler.Help_node ) ) {
+    Per_CPU_Control *cpu;
+
+    cpu = _Thread_Get_CPU( the_thread );
+    _Per_CPU_Acquire( cpu );
+
+    _Chain_Append_unprotected(
+      &cpu->Threads_in_need_for_help,
+      &the_thread->Scheduler.Help_node
+    );
+
+    _Per_CPU_Release( cpu );
+
+    _Thread_Dispatch_request( _Per_CPU_Get(), cpu );
+  }
+
+  _Thread_Scheduler_release_critical( the_thread, &lock_context );
+}