score: Critical fix for SMP

The _Scheduler_SMP_Allocate_processor() and _Thread_Dispatch() exchange
information without locks.  Make sure we use the right load/store
ordering.

cpukit/score/include/rtems/score/cpustdatomic.h

@@ -54,23 +54,13 @@ typedef atomic_uintptr_t Atomic_Pointer;
 typedef atomic_flag Atomic_Flag;
 
 /**
- * @brief the enumeration Atomic_Memory_barrier specifies the detailed regular
+ * @brief Memory order according to ISO/IEC 9899:2011.
- * memory synchronization operations used in the atomic operation API
- * definitions.
  */
 typedef enum {
-  /** no operation orders memory. */
   ATOMIC_ORDER_RELAXED = memory_order_relaxed,
-  /** a load operation performs an acquire operation on the affected memory
-  * location. This flag guarantees that the effects of load operation are
-  * completed before the effects of any later data accesses.
-  */
   ATOMIC_ORDER_ACQUIRE = memory_order_acquire,
-  /** a store operation performs a release operation on the affected memory
+  ATOMIC_ORDER_RELEASE = memory_order_release,
-  * location. This flag guarantee that all effects of all previous data
+  ATOMIC_ORDER_SEQ_CST = memory_order_seq_cst
-  * accesses are completed before the store operation takes place.
-  */
-  ATOMIC_ORDER_RELEASE = memory_order_release
 } Atomic_Order;
 
 

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

@@ -92,12 +92,14 @@ static inline void _Scheduler_SMP_Allocate_processor(
 
     _Thread_Set_CPU( heir, cpu_of_victim );
 
-    /*
-     * FIXME: Here we need atomic store operations with a relaxed memory order.
-     * The _CPU_SMP_Send_interrupt() will ensure that the change can be
-     * observed consistently.
-     */
     cpu_of_victim->heir = heir;
+
+    /*
+     * It is critical that we first update the heir and then the dispatch
+     * necessary so that _Thread_Dispatch() cannot miss an update.
+     */
+    _Atomic_Fence( ATOMIC_ORDER_RELEASE );
+
     cpu_of_victim->dispatch_necessary = true;
 
     if ( cpu_of_victim != cpu_of_executing ) {

cpukit/score/src/threaddispatch.c

@@ -97,9 +97,20 @@ void _Thread_Dispatch( void )
 #else
   while ( per_cpu->dispatch_necessary ) {
 #endif
-    heir = per_cpu->heir;
     per_cpu->dispatch_necessary = false;
+
+#if defined( RTEMS_SMP )
+    /*
+     * It is critical that we first update the dispatch necessary and then the
+     * read the heir so that we don't miss an update by
+     * _Scheduler_SMP_Allocate_processor().
+     */
+    _Atomic_Fence( ATOMIC_ORDER_SEQ_CST );
+#endif
+
+    heir = per_cpu->heir;
     per_cpu->executing = heir;
+
 #if defined( RTEMS_SMP )
     executing->is_executing = false;
     heir->is_executing = true;