[PATCH 26/27] rtems: Fix rtems_scheduler_remove_processor()

[Sebastian Huber]

Return an error status for the following error condition in
rtems_scheduler_remove_processor():

While an attempt is made to remove a processor from a scheduler, while
the processor is the only processor owned by the scheduler, if a thread
exists which uses the scheduler as a helping scheduler, then the
processor shall not be removed.

The reason is that ask for help requests and withdraw node requests are
processed asynchronously in any order.  An ask for help request carried
out on a scheduler without a processor is undefined behaviour.

Update #4544.
---
 cpukit/include/rtems/score/schedulersmpimpl.h | 16 +++---
 cpukit/rtems/src/schedulerremoveprocessor.c   | 50 ++++++++++++++++++-
 2 files changed, 55 insertions(+), 11 deletions(-)

diff --git a/cpukit/include/rtems/score/schedulersmpimpl.h b/cpukit/include/rtems/score/schedulersmpimpl.h
index 97a77121ed..c37f53c8c0 100644
--- a/cpukit/include/rtems/score/schedulersmpimpl.h
+++ b/cpukit/include/rtems/score/schedulersmpimpl.h
@@ -1978,8 +1978,9 @@ static inline Thread_Control *_Scheduler_SMP_Remove_processor(
   victim_owner = _Scheduler_Node_get_owner( victim_node );
 
   if ( !victim_owner->is_idle ) {
-    Thread_Control *victim_idle;
-    Scheduler_Node *idle_node;
+    Thread_Control  *victim_idle;
+    Scheduler_Node  *idle_node;
+    Priority_Control insert_priority;
 
     victim_idle = _Scheduler_Release_idle_thread_if_necessary(
       victim_node,
@@ -1996,13 +1997,10 @@ static inline Thread_Control *_Scheduler_SMP_Remove_processor(
       _Scheduler_SMP_Allocate_processor_exact
     );
 
-    if ( !_Chain_Is_empty( &self->Scheduled ) ) {
-      Priority_Control insert_priority;
-
-      insert_priority = _Scheduler_SMP_Node_priority( victim_node );
-      insert_priority = SCHEDULER_PRIORITY_APPEND( insert_priority );
-      ( *enqueue )( &self->Base, victim_node, insert_priority );
-    }
+    _Assert( !_Chain_Is_empty( &self->Scheduled ) );
+    insert_priority = _Scheduler_SMP_Node_priority( victim_node );
+    insert_priority = SCHEDULER_PRIORITY_APPEND( insert_priority );
+    ( *enqueue )( &self->Base, victim_node, insert_priority );
   } else {
     _Assert( victim_owner == victim_user );
     _Assert( _Scheduler_Node_get_idle( victim_node ) == NULL );
diff --git a/cpukit/rtems/src/schedulerremoveprocessor.c b/cpukit/rtems/src/schedulerremoveprocessor.c
index 79c17bda49..3136a8e0c0 100644
--- a/cpukit/rtems/src/schedulerremoveprocessor.c
+++ b/cpukit/rtems/src/schedulerremoveprocessor.c
@@ -35,7 +35,7 @@ typedef struct {
   rtems_status_code        status;
 } Scheduler_Processor_removal_context;
 
-static bool _Scheduler_Check_processor_removal(
+static bool _Scheduler_Check_processor_not_required(
   Thread_Control *the_thread,
   void           *arg
 )
@@ -68,6 +68,45 @@ static bool _Scheduler_Check_processor_removal(
   _Thread_Wait_release( the_thread, &queue_context );
   return iter_context->status != RTEMS_SUCCESSFUL;
 }
+
+static bool _Scheduler_Check_no_helping(
+  Thread_Control *the_thread,
+  void           *arg
+)
+{
+  Scheduler_Processor_removal_context *iter_context;
+  ISR_lock_Context                     lock_context;
+  const Chain_Node                    *node;
+  const Chain_Node                    *tail;
+
+  if ( the_thread->is_idle ) {
+    return false;
+  }
+
+  iter_context = arg;
+
+  _Thread_State_acquire( the_thread, &lock_context );
+  node = _Chain_Immutable_first( &the_thread->Scheduler.Scheduler_nodes );
+  tail = _Chain_Immutable_tail( &the_thread->Scheduler.Scheduler_nodes );
+
+  do {
+    const Scheduler_Node    *scheduler_node;
+    const Scheduler_Control *scheduler;
+
+    scheduler_node = SCHEDULER_NODE_OF_THREAD_SCHEDULER_NODE( node );
+    scheduler = _Scheduler_Node_get_scheduler( scheduler_node );
+
+    if ( scheduler == iter_context->scheduler ) {
+      iter_context->status = RTEMS_RESOURCE_IN_USE;
+      break;
+    }
+
+    node = _Chain_Immutable_next( node );
+  } while ( node != tail );
+
+  _Thread_State_release( the_thread, &lock_context );
+  return iter_context->status != RTEMS_SUCCESSFUL;
+}
 #endif
 
 rtems_status_code rtems_scheduler_remove_processor(
@@ -116,7 +155,14 @@ rtems_status_code rtems_scheduler_remove_processor(
   _Scheduler_Release_critical( scheduler, &lock_context );
   _ISR_lock_ISR_enable( &lock_context );
 
-  _Thread_Iterate( _Scheduler_Check_processor_removal, &iter_context );
+  _Thread_Iterate( _Scheduler_Check_processor_not_required, &iter_context );
+
+  if (
+    _Processor_mask_Is_zero( &scheduler_context->Processors ) &&
+    iter_context.status == RTEMS_SUCCESSFUL
+  ) {
+    _Thread_Iterate( _Scheduler_Check_no_helping, &iter_context );
+  }
 
   _ISR_lock_ISR_disable( &lock_context );
   _Scheduler_Acquire_critical( scheduler, &lock_context );
-- 
2.26.2