--- old/src/hotspot/share/runtime/objectMonitor.hpp	2019-03-22 16:30:36.805896833 -0400
+++ new/src/hotspot/share/runtime/objectMonitor.hpp	2019-03-22 16:30:36.393896840 -0400
@@ -136,6 +136,7 @@
   };
 
  private:
+  friend class ObjectMonitorHandle;
   friend class ObjectSynchronizer;
   friend class ObjectWaiter;
   friend class VMStructs;
@@ -149,6 +150,8 @@
                         sizeof(volatile markOop) + sizeof(void * volatile) +
                         sizeof(ObjectMonitor *));
  protected:                         // protected for JvmtiRawMonitor
+  // Used by async monitor deflation as a marker in the _owner field:
+  #define DEFLATER_MARKER reinterpret_cast<void*>(-1)
   void *  volatile _owner;          // pointer to owning thread OR BasicLock
   volatile jlong _previous_owner_tid;  // thread id of the previous owner of the monitor
   volatile intptr_t  _recursions;   // recursion count, 0 for first entry
@@ -171,6 +174,13 @@
   volatile jint  _waiters;          // number of waiting threads
  private:
   volatile int _WaitSetLock;        // protects Wait Queue - simple spinlock
+  volatile jint _ref_count;         // ref count for ObjectMonitor*
+  typedef enum {
+    Free = 0,  // Free must be 0 for monitor to be free after memset(..,0,..).
+    New,
+    Old
+  } AllocationState;
+  AllocationState _allocation_state;
 
  public:
   static void Initialize();
@@ -235,12 +245,33 @@
     // TODO-FIXME: merge _count and _waiters.
     // TODO-FIXME: assert _owner == null implies _recursions = 0
     // TODO-FIXME: assert _WaitSet != null implies _count > 0
+    // We do not include _ref_count in the is_busy() check because
+    // _ref_count is for indicating that the ObjectMonitor* is in
+    // use which is orthogonal to whether the ObjectMonitor itself
+    // is in use for a locking operation.
     return _count|_waiters|intptr_t(_owner)|intptr_t(_cxq)|intptr_t(_EntryList);
   }
 
+  // Version of is_busy() that accounts for special values in
+  // _count and _owner when AsyncDeflateIdleMonitors is enabled.
+  intptr_t is_busy_async() const {
+    intptr_t ret_code = _waiters | intptr_t(_cxq) | intptr_t(_EntryList);
+    if (!AsyncDeflateIdleMonitors) {
+      ret_code |= _count | intptr_t(_owner);
+    } else {
+      if (_count > 0) {
+        ret_code |= _count;
+      }
+      if (_owner != DEFLATER_MARKER) {
+        ret_code |= intptr_t(_owner);
+      }
+    }
+    return ret_code;
+  }
+
   intptr_t  is_entered(Thread* current) const;
 
-  void*     owner() const;
+  void*     owner() const;  // Returns NULL if DEFLATER_MARKER is observed.
   void      set_owner(void* owner);
 
   jint      waiters() const;
@@ -285,12 +316,22 @@
   void*     object() const;
   void*     object_addr();
   void      set_object(void* obj);
+  void      set_allocation_state(AllocationState s);
+  AllocationState allocation_state() const;
+  bool      is_free() const;
+  bool      is_active() const;
+  bool      is_old() const;
+  bool      is_new() const;
+  void      dec_ref_count();
+  void      inc_ref_count();
+  jint      ref_count() const;
 
   bool      check(TRAPS);       // true if the thread owns the monitor.
   void      check_slow(TRAPS);
   void      clear();
+  void      clear_using_JT();
 
-  void      enter(TRAPS);
+  bool      enter(TRAPS);  // Returns false if monitor is being async deflated and caller should retry locking the object.
   void      exit(bool not_suspended, TRAPS);
   void      wait(jlong millis, bool interruptable, TRAPS);
   void      notify(TRAPS);
@@ -298,7 +339,7 @@
 
 // Use the following at your own risk
   intptr_t  complete_exit(TRAPS);
-  void      reenter(intptr_t recursions, TRAPS);
+  bool      reenter(intptr_t recursions, TRAPS);  // Returns false if monitor is being async deflated and caller should retry locking the object.
 
  private:
   void      AddWaiter(ObjectWaiter * waiter);
@@ -313,6 +354,27 @@
   int       TrySpin(Thread * Self);
   void      ExitEpilog(Thread * Self, ObjectWaiter * Wakee);
   bool      ExitSuspendEquivalent(JavaThread * Self);
+  void      install_displaced_markword_in_object();
+};
+
+// A helper object for managing an ObjectMonitor*'s ref_count. There
+// are special safety considerations when async deflation is used.
+class ObjectMonitorHandle : public StackObj {
+ private:
+  ObjectMonitor * _om_ptr;
+ public:
+  ObjectMonitorHandle() { _om_ptr = NULL; }
+  ~ObjectMonitorHandle();
+
+  ObjectMonitor * om_ptr() const { return _om_ptr; }
+  // Save the ObjectMonitor* associated with the specified markOop and
+  // increment the ref_count.
+  bool save_om_ptr(oop object, markOop mark);
+
+  // For internal used by ObjectSynchronizer::monitors_iterate().
+  ObjectMonitorHandle(ObjectMonitor * _om_ptr);
+  // For internal use by ObjectSynchronizer::inflate().
+  void set_om_ptr(ObjectMonitor * om_ptr);
 };
 
 #endif // SHARE_RUNTIME_OBJECTMONITOR_HPP