--- a/src/hotspot/share/classfile/classLoaderDataGraph.cpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/classfile/classLoaderDataGraph.cpp	Tue Oct 16 13:14:18 2018 +0200
@@ -231,14 +231,14 @@
 }
 
 void ClassLoaderDataGraph::cld_do(CLDClosure* cl) {
-  assert_locked_or_safepoint(ClassLoaderDataGraph_lock);
+  assert_locked_or_safepoint_weak(ClassLoaderDataGraph_lock);
   for (ClassLoaderData* cld = _head;  cld != NULL; cld = cld->_next) {
     cl->do_cld(cld);
   }
 }
 
 void ClassLoaderDataGraph::cld_unloading_do(CLDClosure* cl) {
-  assert_locked_or_safepoint(ClassLoaderDataGraph_lock);
+  assert_locked_or_safepoint_weak(ClassLoaderDataGraph_lock);
   // Only walk the head until any clds not purged from prior unloading
   // (CMS doesn't purge right away).
   for (ClassLoaderData* cld = _unloading; cld != _saved_unloading; cld = cld->next()) {
@@ -248,7 +248,7 @@
 }
 
 void ClassLoaderDataGraph::roots_cld_do(CLDClosure* strong, CLDClosure* weak) {
-  assert_locked_or_safepoint(ClassLoaderDataGraph_lock);
+  assert_locked_or_safepoint_weak(ClassLoaderDataGraph_lock);
   for (ClassLoaderData* cld = _head;  cld != NULL; cld = cld->_next) {
     CLDClosure* closure = cld->keep_alive() ? strong : weak;
     if (closure != NULL) {
@@ -258,7 +258,7 @@
 }
 
 void ClassLoaderDataGraph::always_strong_cld_do(CLDClosure* cl) {
-  assert_locked_or_safepoint(ClassLoaderDataGraph_lock);
+  assert_locked_or_safepoint_weak(ClassLoaderDataGraph_lock);
   if (ClassUnloading) {
     roots_cld_do(cl, NULL);
   } else {

--- a/src/hotspot/share/gc/z/zCollectedHeap.cpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/gc/z/zCollectedHeap.cpp	Tue Oct 16 13:14:18 2018 +0200
@@ -23,6 +23,7 @@
 
 #include "precompiled.hpp"
 #include "gc/shared/gcHeapSummary.hpp"
+#include "gc/shared/suspendibleThreadSet.hpp"
 #include "gc/z/zCollectedHeap.hpp"
 #include "gc/z/zGlobals.hpp"
 #include "gc/z/zHeap.inline.hpp"
@@ -295,6 +296,14 @@
                              reserved_region().start() + max_capacity_in_words);
 }
 
+void ZCollectedHeap::safepoint_synchronize_begin() {
+  SuspendibleThreadSet::synchronize();
+}
+
+void ZCollectedHeap::safepoint_synchronize_end() {
+  SuspendibleThreadSet::desynchronize();
+}
+
 void ZCollectedHeap::prepare_for_verify() {
   // Does nothing
 }

--- a/src/hotspot/share/gc/z/zCollectedHeap.hpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/gc/z/zCollectedHeap.hpp	Tue Oct 16 13:14:18 2018 +0200
@@ -117,6 +117,9 @@
 
   virtual VirtualSpaceSummary create_heap_space_summary();
 
+  virtual void safepoint_synchronize_begin();
+  virtual void safepoint_synchronize_end();
+
   virtual void print_on(outputStream* st) const;
   virtual void print_on_error(outputStream* st) const;
   virtual void print_extended_on(outputStream* st) const;

--- a/src/hotspot/share/gc/z/zDriver.cpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/gc/z/zDriver.cpp	Tue Oct 16 13:14:18 2018 +0200
@@ -336,7 +336,7 @@
   // Phase 2: Concurrent Mark
   {
     ZStatTimer timer(ZPhaseConcurrentMark);
-    ZHeap::heap()->mark();
+    ZHeap::heap()->mark(true /* initial */);
   }
 
   // Phase 3: Pause Mark End
@@ -345,7 +345,7 @@
     while (!vm_operation(&cl)) {
       // Phase 3.5: Concurrent Mark Continue
       ZStatTimer timer(ZPhaseConcurrentMarkContinue);
-      ZHeap::heap()->mark();
+      ZHeap::heap()->mark(false /* initial */);
     }
   }
 

--- a/src/hotspot/share/gc/z/zHeap.cpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/gc/z/zHeap.cpp	Tue Oct 16 13:14:18 2018 +0200
@@ -296,8 +296,8 @@
   ZStatHeap::set_at_mark_start(capacity(), used());
 }
 
-void ZHeap::mark() {
-  _mark.mark();
+void ZHeap::mark(bool initial) {
+  _mark.mark(initial);
 }
 
 void ZHeap::mark_flush_and_free(Thread* thread) {

--- a/src/hotspot/share/gc/z/zHeap.hpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/gc/z/zHeap.hpp	Tue Oct 16 13:14:18 2018 +0200
@@ -133,7 +133,7 @@
   bool is_object_strongly_live(uintptr_t addr) const;
   template <bool finalizable, bool publish> void mark_object(uintptr_t addr);
   void mark_start();
-  void mark();
+  void mark(bool initial);
   void mark_flush_and_free(Thread* thread);
   bool mark_end();
 

--- a/src/hotspot/share/gc/z/zHeapIterator.cpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/gc/z/zHeapIterator.cpp	Tue Oct 16 13:14:18 2018 +0200
@@ -172,8 +172,10 @@
     // this the user would have expected to see ObjectFree events for
     // unreachable objects in the tag map.
     ZRootsIterator roots;
+    ZConcurrentRootsIterator concurrent_roots(false /* marking */);
     ZHeapIteratorRootOopClosure root_cl(this);
     roots.oops_do(&root_cl, true /* visit_jvmti_weak_export */);
+    concurrent_roots.oops_do(&root_cl);
   }
 
   // Drain stack

--- a/src/hotspot/share/gc/z/zMark.cpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/gc/z/zMark.cpp	Tue Oct 16 13:14:18 2018 +0200
@@ -615,6 +615,34 @@
   stacks->free(&_allocator);
 }
 
+class ZMarkConcurrentRootsIteratorClosure : public ZRootsIteratorClosure {
+public:
+  virtual void do_oop(oop* p) {
+    ZBarrier::mark_barrier_on_oop_field(p, false /* finalizable */);
+  }
+
+  virtual void do_oop(narrowOop* p) {
+    ShouldNotReachHere();
+  }
+};
+
+
+class ZMarkConcurrentRootsTask : public ZTask {
+private:
+  ZConcurrentRootsIterator            _roots;
+  ZMarkConcurrentRootsIteratorClosure _cl;
+
+public:
+  ZMarkConcurrentRootsTask(ZMark* mark) :
+      ZTask("ZMarkConcurrentRootsTask"),
+      _roots(true /* marking */),
+      _cl() {}
+
+  virtual void work() {
+    _roots.oops_do(&_cl);
+  }
+};
+
 class ZMarkTask : public ZTask {
 private:
   ZMark* const   _mark;
@@ -637,7 +665,12 @@
   }
 };
 
-void ZMark::mark() {
+void ZMark::mark(bool initial) {
+  if (initial) {
+    ZMarkConcurrentRootsTask task(this);
+    _workers->run_concurrent(&task);
+  }
+
   ZMarkTask task(this);
   _workers->run_concurrent(&task);
 }

--- a/src/hotspot/share/gc/z/zMark.hpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/gc/z/zMark.hpp	Tue Oct 16 13:14:18 2018 +0200
@@ -108,7 +108,7 @@
   template <bool finalizable, bool publish> void mark_object(uintptr_t addr);
 
   void start();
-  void mark();
+  void mark(bool initial);
   bool end();
 
   void flush_and_free();

--- a/src/hotspot/share/gc/z/zRootsIterator.cpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/gc/z/zRootsIterator.cpp	Tue Oct 16 13:14:18 2018 +0200
@@ -28,6 +28,7 @@
 #include "code/codeCache.hpp"
 #include "compiler/oopMap.hpp"
 #include "gc/shared/oopStorageParState.inline.hpp"
+#include "gc/shared/suspendibleThreadSet.hpp"
 #include "gc/z/zGlobals.hpp"
 #include "gc/z/zNMethodTable.hpp"
 #include "gc/z/zOopClosures.inline.hpp"
@@ -52,16 +53,20 @@
 static const ZStatSubPhase ZSubPhasePauseRoots("Pause Roots");
 static const ZStatSubPhase ZSubPhasePauseRootsTeardown("Pause Roots Teardown");
 static const ZStatSubPhase ZSubPhasePauseRootsUniverse("Pause Roots Universe");
-static const ZStatSubPhase ZSubPhasePauseRootsJNIHandles("Pause Roots JNIHandles");
 static const ZStatSubPhase ZSubPhasePauseRootsObjectSynchronizer("Pause Roots ObjectSynchronizer");
 static const ZStatSubPhase ZSubPhasePauseRootsManagement("Pause Roots Management");
 static const ZStatSubPhase ZSubPhasePauseRootsJVMTIExport("Pause Roots JVMTIExport");
 static const ZStatSubPhase ZSubPhasePauseRootsJVMTIWeakExport("Pause Roots JVMTIWeakExport");
 static const ZStatSubPhase ZSubPhasePauseRootsSystemDictionary("Pause Roots SystemDictionary");
-static const ZStatSubPhase ZSubPhasePauseRootsClassLoaderDataGraph("Pause Roots ClassLoaderDataGraph");
 static const ZStatSubPhase ZSubPhasePauseRootsThreads("Pause Roots Threads");
 static const ZStatSubPhase ZSubPhasePauseRootsCodeCache("Pause Roots CodeCache");
 
+static const ZStatSubPhase ZSubPhaseConcurrentRootsSetup("Concurrent Roots Setup");
+static const ZStatSubPhase ZSubPhaseConcurrentRoots("Concurrent Roots");
+static const ZStatSubPhase ZSubPhaseConcurrentRootsTeardown("Concurrent Roots Teardown");
+static const ZStatSubPhase ZSubPhaseConcurrentRootsJNIHandles("Concurrent Roots JNIHandles");
+static const ZStatSubPhase ZSubPhaseConcurrentRootsClassLoaderDataGraph("Concurrent Roots ClassLoaderDataGraph");
+
 static const ZStatSubPhase ZSubPhasePauseWeakRootsSetup("Pause Weak Roots Setup");
 static const ZStatSubPhase ZSubPhasePauseWeakRoots("Pause Weak Roots");
 static const ZStatSubPhase ZSubPhasePauseWeakRootsTeardown("Pause Weak Roots Teardown");
@@ -128,21 +133,17 @@
 }
 
 ZRootsIterator::ZRootsIterator() :
-    _jni_handles_iter(JNIHandles::global_handles()),
     _universe(this),
     _object_synchronizer(this),
     _management(this),
     _jvmti_export(this),
     _jvmti_weak_export(this),
     _system_dictionary(this),
-    _jni_handles(this),
-    _class_loader_data_graph(this),
     _threads(this),
     _code_cache(this) {
   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();
@@ -163,11 +164,6 @@
   Universe::oops_do(cl);
 }
 
-void ZRootsIterator::do_jni_handles(ZRootsIteratorClosure* cl) {
-  ZStatTimer timer(ZSubPhasePauseRootsJNIHandles);
-  _jni_handles_iter.oops_do(cl);
-}
-
 void ZRootsIterator::do_object_synchronizer(ZRootsIteratorClosure* cl) {
   ZStatTimer timer(ZSubPhasePauseRootsObjectSynchronizer);
   ObjectSynchronizer::oops_do(cl);
@@ -194,12 +190,6 @@
   SystemDictionary::oops_do(cl);
 }
 
-void ZRootsIterator::do_class_loader_data_graph(ZRootsIteratorClosure* cl) {
-  ZStatTimer timer(ZSubPhasePauseRootsClassLoaderDataGraph);
-  CLDToOopClosure cld_cl(cl);
-  ClassLoaderDataGraph::cld_do(&cld_cl);
-}
-
 void ZRootsIterator::do_threads(ZRootsIteratorClosure* cl) {
   ZStatTimer timer(ZSubPhasePauseRootsThreads);
   ResourceMark rm;
@@ -218,8 +208,6 @@
   _management.oops_do(cl);
   _jvmti_export.oops_do(cl);
   _system_dictionary.oops_do(cl);
-  _jni_handles.oops_do(cl);
-  _class_loader_data_graph.oops_do(cl);
   _threads.oops_do(cl);
   _code_cache.oops_do(cl);
   if (visit_jvmti_weak_export) {
@@ -227,6 +215,43 @@
   }
 }
 
+ZConcurrentRootsIterator::ZConcurrentRootsIterator(bool marking) :
+    _marking(marking),
+    _sts_joiner(marking /* active */),
+    _jni_handles_iter(JNIHandles::global_handles()),
+    _jni_handles(this),
+    _class_loader_data_graph(this) {
+  ZStatTimer timer(ZSubPhaseConcurrentRootsSetup);
+  if (_marking) {
+    ClassLoaderDataGraph_lock->lock();
+    ClassLoaderDataGraph::clear_claimed_marks();
+  }
+}
+
+ZConcurrentRootsIterator::~ZConcurrentRootsIterator() {
+  ZStatTimer timer(ZSubPhaseConcurrentRootsTeardown);
+  if (_marking) {
+    ClassLoaderDataGraph_lock->unlock();
+  }
+}
+
+void ZConcurrentRootsIterator::do_jni_handles(ZRootsIteratorClosure* cl) {
+  ZStatTimer timer(ZSubPhaseConcurrentRootsJNIHandles);
+  _jni_handles_iter.oops_do(cl);
+}
+
+void ZConcurrentRootsIterator::do_class_loader_data_graph(ZRootsIteratorClosure* cl) {
+  ZStatTimer timer(ZSubPhaseConcurrentRootsClassLoaderDataGraph);
+  CLDToOopClosure cld_cl(cl, _marking /* must_claim */);
+  ClassLoaderDataGraph::cld_do(&cld_cl);
+}
+
+void ZConcurrentRootsIterator::oops_do(ZRootsIteratorClosure* cl) {
+  ZStatTimer timer(ZSubPhaseConcurrentRoots);
+  _jni_handles.oops_do(cl);
+  _class_loader_data_graph.oops_do(cl);
+}
+
 ZWeakRootsIterator::ZWeakRootsIterator() :
     _jvmti_weak_export(this),
     _jfr_weak(this) {

--- a/src/hotspot/share/gc/z/zRootsIterator.hpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/gc/z/zRootsIterator.hpp	Tue Oct 16 13:14:18 2018 +0200
@@ -25,6 +25,7 @@
 #define SHARE_GC_Z_ZROOTSITERATOR_HPP
 
 #include "gc/shared/oopStorageParState.hpp"
+#include "gc/shared/suspendibleThreadSet.hpp"
 #include "memory/allocation.hpp"
 #include "memory/iterator.hpp"
 #include "runtime/thread.hpp"
@@ -37,8 +38,7 @@
   }
 };
 
-typedef OopStorage::ParState<false /* concurrent */, false /* is_const */> ZOopStorageIterator;
-typedef OopStorage::ParState<true /* concurrent */, false /* is_const */>  ZConcurrentOopStorageIterator;
+typedef OopStorage::ParState<true /* concurrent */, false /* is_const */> ZOopStorageIterator;
 
 template <typename T, void (T::*F)(ZRootsIteratorClosure*)>
 class ZSerialOopsDo {
@@ -86,29 +86,23 @@
 
 class ZRootsIterator {
 private:
-  ZOopStorageIterator _jni_handles_iter;
-
   void do_universe(ZRootsIteratorClosure* cl);
-  void do_jni_handles(ZRootsIteratorClosure* cl);
   void do_object_synchronizer(ZRootsIteratorClosure* cl);
   void do_management(ZRootsIteratorClosure* cl);
   void do_jvmti_export(ZRootsIteratorClosure* cl);
   void do_jvmti_weak_export(ZRootsIteratorClosure* cl);
   void do_system_dictionary(ZRootsIteratorClosure* cl);
-  void do_class_loader_data_graph(ZRootsIteratorClosure* cl);
   void do_threads(ZRootsIteratorClosure* cl);
   void do_code_cache(ZRootsIteratorClosure* cl);
 
-  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_universe>                  _universe;
-  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_object_synchronizer>       _object_synchronizer;
-  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_management>                _management;
-  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_jvmti_export>              _jvmti_export;
-  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_jvmti_weak_export>         _jvmti_weak_export;
-  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_system_dictionary>         _system_dictionary;
-  ZParallelOopsDo<ZRootsIterator, &ZRootsIterator::do_jni_handles>             _jni_handles;
-  ZParallelOopsDo<ZRootsIterator, &ZRootsIterator::do_class_loader_data_graph> _class_loader_data_graph;
-  ZParallelOopsDo<ZRootsIterator, &ZRootsIterator::do_threads>                 _threads;
-  ZParallelOopsDo<ZRootsIterator, &ZRootsIterator::do_code_cache>              _code_cache;
+  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_universe>            _universe;
+  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_object_synchronizer> _object_synchronizer;
+  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_management>          _management;
+  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_jvmti_export>        _jvmti_export;
+  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_jvmti_weak_export>   _jvmti_weak_export;
+  ZSerialOopsDo<ZRootsIterator, &ZRootsIterator::do_system_dictionary>   _system_dictionary;
+  ZParallelOopsDo<ZRootsIterator, &ZRootsIterator::do_threads>           _threads;
+  ZParallelOopsDo<ZRootsIterator, &ZRootsIterator::do_code_cache>        _code_cache;
 
 public:
   ZRootsIterator();
@@ -117,6 +111,25 @@
   void oops_do(ZRootsIteratorClosure* cl, bool visit_jvmti_weak_export = false);
 };
 
+class ZConcurrentRootsIterator {
+private:
+  const bool                 _marking;
+  SuspendibleThreadSetJoiner _sts_joiner;
+  ZOopStorageIterator        _jni_handles_iter;
+
+  void do_jni_handles(ZRootsIteratorClosure* cl);
+  void do_class_loader_data_graph(ZRootsIteratorClosure* cl);
+
+  ZParallelOopsDo<ZConcurrentRootsIterator, &ZConcurrentRootsIterator::do_jni_handles>             _jni_handles;
+  ZParallelOopsDo<ZConcurrentRootsIterator, &ZConcurrentRootsIterator::do_class_loader_data_graph> _class_loader_data_graph;
+
+public:
+  ZConcurrentRootsIterator(bool marking);
+  ~ZConcurrentRootsIterator();
+
+  void oops_do(ZRootsIteratorClosure* cl);
+};
+
 class ZWeakRootsIterator {
 private:
   void do_jvmti_weak_export(BoolObjectClosure* is_alive, ZRootsIteratorClosure* cl);
@@ -135,9 +148,9 @@
 
 class ZConcurrentWeakRootsIterator {
 private:
-  ZConcurrentOopStorageIterator _vm_weak_handles_iter;
-  ZConcurrentOopStorageIterator _jni_weak_handles_iter;
-  ZConcurrentOopStorageIterator _string_table_iter;
+  ZOopStorageIterator _vm_weak_handles_iter;
+  ZOopStorageIterator _jni_weak_handles_iter;
+  ZOopStorageIterator _string_table_iter;
 
   void do_vm_weak_handles(ZRootsIteratorClosure* cl);
   void do_jni_weak_handles(ZRootsIteratorClosure* cl);

--- a/src/hotspot/share/runtime/mutexLocker.cpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/runtime/mutexLocker.cpp	Tue Oct 16 13:14:18 2018 +0200
@@ -166,6 +166,16 @@
   fatal("must own lock %s", lock->name());
 }
 
+// a weaker assertion than the above
+void assert_locked_or_safepoint_weak(const Monitor * lock) {
+  if (IgnoreLockingAssertions) return;
+  assert(lock != NULL, "Need non-NULL lock");
+  if (lock->is_locked()) return;
+  if (SafepointSynchronize::is_at_safepoint()) return;
+  if (!Universe::is_fully_initialized()) return;
+  fatal("must own lock %s", lock->name());
+}
+
 // a stronger assertion than the above
 void assert_lock_strong(const Monitor * lock) {
   if (IgnoreLockingAssertions) return;

--- a/src/hotspot/share/runtime/mutexLocker.hpp	Tue Oct 16 13:43:04 2018 +0200
+++ b/src/hotspot/share/runtime/mutexLocker.hpp	Tue Oct 16 13:14:18 2018 +0200
@@ -199,9 +199,11 @@
 // for debugging: check that we're already owning this lock (or are at a safepoint)
 #ifdef ASSERT
 void assert_locked_or_safepoint(const Monitor * lock);
+void assert_locked_or_safepoint_weak(const Monitor * lock);
 void assert_lock_strong(const Monitor * lock);
 #else
 #define assert_locked_or_safepoint(lock)
+#define assert_locked_or_safepoint_weak(lock)
 #define assert_lock_strong(lock)
 #endif