--- old/src/share/vm/gc/shenandoah/shenandoahConcurrentThread.cpp	2017-10-09 11:48:33.090331126 +0200
+++ new/src/share/vm/gc/shenandoah/shenandoahConcurrentThread.cpp	2017-10-09 11:48:32.988330875 +0200
@@ -162,6 +162,18 @@
   TraceCollectorStats tcs(heap->monitoring_support()->concurrent_collection_counters());
   TraceMemoryManagerStats tmms(false, GCCause::_no_cause_specified);
 
+  // Mark requires clean bitmaps. Clear them here, before diving into STW.
+  // There is a potential race from this moment on to TAMS reset in init mark: the bitmaps
+  // would be clear, but TAMS not yet updated.
+  {
+    GCTraceTime(Info, gc) time("Concurrent cleanup", gc_timer, GCCause::_no_gc, true);
+    ShenandoahGCPhase phase(ShenandoahPhaseTimings::conc_cleanup);
+    ShenandoahGCPhase phase_reset(ShenandoahPhaseTimings::conc_cleanup_reset_bitmaps);
+    WorkGang *workers = heap->workers();
+    ShenandoahPushWorkerScope scope(workers, ConcGCThreads);
+    heap->reset_mark_bitmap(workers);
+  }
+
   // Start initial mark under STW:
   {
     // Workers are setup by VM_ShenandoahInitMark
@@ -312,6 +324,17 @@
       VM_ShenandoahFinalUpdateRefs final_update_refs;
       VMThread::execute(&final_update_refs);
     }
+
+    if (do_it) {
+      GCTraceTime(Info, gc) time("Concurrent cleanup", gc_timer, GCCause::_no_gc, true);
+      ShenandoahGCPhase phase(ShenandoahPhaseTimings::conc_cleanup);
+      ShenandoahGCPhase phase_recycle(ShenandoahPhaseTimings::conc_cleanup_recycle);
+      heap->recycle_trash();
+    }
+
+    // Allocations happen during bitmap cleanup, record peak after the phase:
+    heap->shenandoahPolicy()->record_peak_occupancy();
+
   } else {
     // If update-refs were skipped, need to do another verification pass after evacuation.
     if (ShenandoahVerify && !check_cancellation()) {
@@ -327,26 +350,6 @@
     reset_full_gc();
   }
 
-  {
-    GCTraceTime(Info, gc) time("Concurrent cleanup", gc_timer, GCCause::_no_gc, true);
-    ShenandoahGCPhase phase(ShenandoahPhaseTimings::conc_cleanup);
-
-    {
-      ShenandoahGCPhase phase_recycle(ShenandoahPhaseTimings::conc_cleanup_recycle);
-      heap->recycle_trash();
-    }
-
-    {
-      ShenandoahGCPhase phase_reset(ShenandoahPhaseTimings::conc_cleanup_reset_bitmaps);
-      WorkGang *workers = heap->workers();
-      ShenandoahPushWorkerScope scope(workers, ConcGCThreads);
-      heap->reset_next_mark_bitmap(workers);
-    }
-  }
-
-  // Allocations happen during bitmap cleanup, record peak after the phase:
-  heap->shenandoahPolicy()->record_peak_occupancy();
-
   // Cycle is complete
   heap->shenandoahPolicy()->record_cycle_end();