8021809: Partitioning based on eden sampling during allocation not reset correctly

Reviewed-by: ysr, hiroshi

hotspot/src/share/vm/gc_implementation/concurrentMarkSweep/concurrentMarkSweepGeneration.cpp

@@ -5478,11 +5478,12 @@ CMSParMarkTask::do_young_space_rescan(uint worker_id,
   HandleMark   hm;
 
   SequentialSubTasksDone* pst = space->par_seq_tasks();
-  assert(pst->valid(), "Uninitialized use?");
 
   uint nth_task = 0;
   uint n_tasks  = pst->n_tasks();
 
+  if (n_tasks > 0) {
+    assert(pst->valid(), "Uninitialized use?");
     HeapWord *start, *end;
     while (!pst->is_task_claimed(/* reference */ nth_task)) {
       // We claimed task # nth_task; compute its boundaries.
@@ -5512,6 +5513,7 @@ CMSParMarkTask::do_young_space_rescan(uint worker_id,
       space->par_oop_iterate(mr, cl);
     }
     pst->all_tasks_completed();
+  }
 }
 
 void
@@ -5788,7 +5790,7 @@ initialize_sequential_subtasks_for_young_gen_rescan(int n_threads) {
   DefNewGeneration* dng = (DefNewGeneration*)_young_gen;
 
   // Eden space
-  {
+  if (!dng->eden()->is_empty()) {
     SequentialSubTasksDone* pst = dng->eden()->par_seq_tasks();
     assert(!pst->valid(), "Clobbering existing data?");
     // Each valid entry in [0, _eden_chunk_index) represents a task.