--- a/src/hotspot/share/gc/g1/g1ConcurrentMarkThread.cpp	Wed May 08 11:59:14 2019 +0200
+++ b/src/hotspot/share/gc/g1/g1ConcurrentMarkThread.cpp	Wed May 08 06:40:45 2019 -0400
@@ -268,6 +268,7 @@
 
       {
         G1ConcPhase p(G1ConcurrentPhase::CLEAR_CLAIMED_MARKS, this);
+        MutexLocker ml(ClassLoaderDataGraph_lock);
         ClassLoaderDataGraph::clear_claimed_marks();
       }