--- a/src/hotspot/share/gc/z/zMark.cpp	Tue Jul 02 12:24:26 2019 +0200
+++ b/src/hotspot/share/gc/z/zMark.cpp	Mon Jun 10 13:04:12 2019 +0200
@@ -22,6 +22,7 @@
  */
 
 #include "precompiled.hpp"
+#include "classfile/classLoaderDataGraph.hpp"
 #include "gc/z/zBarrier.inline.hpp"
 #include "gc/z/zMark.inline.hpp"
 #include "gc/z/zMarkCache.inline.hpp"
@@ -632,14 +633,23 @@
 
 class ZMarkConcurrentRootsTask : public ZTask {
 private:
+  SuspendibleThreadSetJoiner          _sts_joiner;
   ZConcurrentRootsIterator            _roots;
   ZMarkConcurrentRootsIteratorClosure _cl;
 
 public:
   ZMarkConcurrentRootsTask(ZMark* mark) :
       ZTask("ZMarkConcurrentRootsTask"),
-      _roots(true /* marking */),
-      _cl() {}
+      _sts_joiner(true /* active */),
+      _roots(ClassLoaderData::_claim_strong),
+      _cl() {
+    ClassLoaderDataGraph_lock->lock();
+    ClassLoaderDataGraph::clear_claimed_marks();
+  }
+
+  ~ZMarkConcurrentRootsTask() {
+    ClassLoaderDataGraph_lock->unlock();
+  }
 
   virtual void work() {
     _roots.oops_do(&_cl);