diff --git a/src/hotspot/share/gc/z/zRootsIterator.cpp b/src/hotspot/share/gc/z/zRootsIterator.cpp
index f29cfdd..317c68d 100644
--- a/src/hotspot/share/gc/z/zRootsIterator.cpp
+++ b/src/hotspot/share/gc/z/zRootsIterator.cpp
@@ -143,7 +143,6 @@ ZRootsIterator::ZRootsIterator() :
   assert(SafepointSynchronize::is_at_safepoint(), "Should be at safepoint");
   ZStatTimer timer(ZSubPhasePauseRootsSetup);
   Threads::change_thread_claim_parity();
-  ClassLoaderDataGraph::clear_claimed_marks();
   COMPILER2_PRESENT(DerivedPointerTable::clear());
   CodeCache::gc_prologue();
   ZNMethodTable::gc_prologue();
@@ -240,10 +239,13 @@ ZConcurrentRootsIterator::ZConcurrentRootsIterator() :
     _jni_handles(this),
     _class_loader_data_graph(this) {
   ZStatTimer timer(ZSubPhaseConcurrentRootsSetup);
+  ClassLoaderDataGraph_lock->lock();
+  ClassLoaderDataGraph::clear_claimed_marks();
 }
 
 ZConcurrentRootsIterator::~ZConcurrentRootsIterator() {
   ZStatTimer timer(ZSubPhaseConcurrentRootsTeardown);
+  ClassLoaderDataGraph_lock->unlock();
 }
 
 void ZConcurrentRootsIterator::do_jni_handles(OopClosure* cl) {