--- old/src/hotspot/share/gc/shenandoah/shenandoahTraversalGC.cpp	2018-06-19 21:10:11.436539153 +0200
+++ new/src/hotspot/share/gc/shenandoah/shenandoahTraversalGC.cpp	2018-06-19 21:10:11.242541529 +0200
@@ -243,8 +243,17 @@
       rp = _heap->ref_processor();
     }
 
-    // Step 1: Process ordinary GC roots.
+    // Step 0: Drain outstanding SATB queues.
+    // NOTE: we piggy-back draining of remaining thread SATB buffers on the final root scan below.
     ShenandoahTraversalSATBBufferClosure satb_cl(q);
+    {
+      // Process remaining finished SATB buffers.
+      SATBMarkQueueSet& satb_mq_set = ShenandoahBarrierSet::satb_mark_queue_set();
+      while (satb_mq_set.apply_closure_to_completed_buffer(&satb_cl));
+      // Process remaining threads SATB buffers below.
+    }
+
+    // Step 1: Process ordinary GC roots.
     if (!_heap->is_degenerated_gc_in_progress()) {
       ShenandoahTraversalClosure roots_cl(q, rp);
       CLDToOopClosure cld_cl(&roots_cl);