--- old/src/hotspot/share/runtime/objectMonitor.cpp	2019-04-18 22:08:10.880306187 -0400
+++ new/src/hotspot/share/runtime/objectMonitor.cpp	2019-04-18 22:08:10.152306199 -0400
@@ -238,7 +238,7 @@
 // -----------------------------------------------------------------------------
 // Enter support
 
-void ObjectMonitor::enter(TRAPS) {
+bool ObjectMonitor::enter(TRAPS) {
   // The following code is ordered to check the most common cases first
   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
   Thread * const Self = THREAD;
@@ -248,13 +248,13 @@
     // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
     assert(_recursions == 0, "invariant");
     assert(_owner == Self, "invariant");
-    return;
+    return true;
   }
 
   if (cur == Self) {
     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
     _recursions++;
-    return;
+    return true;
   }
 
   if (Self->is_lock_owned ((address)cur)) {
@@ -263,7 +263,7 @@
     // Commute owner from a thread-specific on-stack BasicLockObject address to
     // a full-fledged "Thread *".
     _owner = Self;
-    return;
+    return true;
   }
 
   // We've encountered genuine contention.
@@ -284,7 +284,7 @@
            ", encoded this=" INTPTR_FORMAT, p2i(((oop)object())->mark()),
            p2i(markOopDesc::encode(this)));
     Self->_Stalled = 0;
-    return;
+    return true;
   }
 
   assert(_owner != Self, "invariant");
@@ -293,12 +293,22 @@
   JavaThread * jt = (JavaThread *) Self;
   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
   assert(jt->thread_state() != _thread_blocked, "invariant");
-  assert(this->object() != NULL, "invariant");
-  assert(_contentions >= 0, "invariant");
+  assert(AsyncDeflateIdleMonitors || this->object() != NULL, "invariant");
+  assert(AsyncDeflateIdleMonitors || _contentions >= 0, "invariant");
 
-  // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
+  // Prevent deflation. See ObjectSynchronizer::deflate_monitor() and is_busy().
   // Ensure the object-monitor relationship remains stable while there's contention.
-  Atomic::inc(&_contentions);
+  const jint contentions = Atomic::add(1, &_contentions);
+  if (contentions <= 0 && _owner == DEFLATER_MARKER) {
+    // Async deflation is in progress. Attempt to restore the
+    // header/dmw to the object's header so that we only retry once
+    // if the deflater thread happens to be slow.
+    const oop obj = (oop) object();
+    install_displaced_markword_in_object(obj);
+    Self->_Stalled = 0;
+    return false;  // Caller should retry. Never mind about _contentions as this monitor has been deflated.
+  }
+  // The deflater thread will not deflate this monitor and the monitor is contended, continue.
 
   JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
   EventJavaMonitorEnter event;
@@ -360,7 +370,7 @@
   }
 
   Atomic::dec(&_contentions);
-  assert(_contentions >= 0, "invariant");
+  assert(AsyncDeflateIdleMonitors || _contentions >= 0, "invariant");
   Self->_Stalled = 0;
 
   // Must either set _recursions = 0 or ASSERT _recursions == 0.
@@ -396,6 +406,7 @@
     event.commit();
   }
   OM_PERFDATA_OP(ContendedLockAttempts, inc());
+  return true;
 }
 
 // Caveat: TryLock() is not necessarily serializing if it returns failure.
@@ -417,6 +428,85 @@
   return -1;
 }
 
+// Install the displaced mark word (dmw) of a deflating ObjectMonitor
+// into the header of the object associated with the monitor. This
+// idempotent method is called by a thread that is deflating a
+// monitor and by other threads that have detected a race with the
+// deflation process.
+void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
+  // This function must only be called when (owner == DEFLATER_MARKER
+  // && contentions <= 0), but we can't guarantee that here because
+  // those values could change when the ObjectMonitor gets moved from
+  // the global free list to a per-thread free list.
+
+  guarantee(obj != NULL, "must be non-NULL");
+  if (object() != obj) {
+    // ObjectMonitor's object ref no longer refers to the target object
+    // so the object's header has already been restored.
+    return;
+  }
+
+  markOop dmw = header();
+  if (dmw == NULL) {
+    // ObjectMonitor's header/dmw has been cleared by the deflating
+    // thread so the object's header has already been restored.
+    return;
+  }
+
+  // A non-NULL dmw has to be either neutral (not locked and not marked)
+  // or is already participating in this restoration protocol.
+  assert(dmw->is_neutral() || (dmw->is_marked() && dmw->hash() == 0),
+         "failed precondition: dmw=" INTPTR_FORMAT, p2i(dmw));
+
+  markOop marked_dmw = NULL;
+  if (!dmw->is_marked() && dmw->hash() == 0) {
+    // This dmw has not yet started the restoration protocol so we
+    // mark a copy of the dmw to begin the protocol.
+    // Note: A dmw with a hashcode does not take this code path.
+    marked_dmw = dmw->set_marked();
+
+    // All of the callers to this function can be racing with each
+    // other trying to update the _header field.
+    dmw = (markOop) Atomic::cmpxchg(marked_dmw, &_header, dmw);
+    if (dmw == NULL) {
+      // ObjectMonitor's header/dmw has been cleared by the deflating
+      // thread so the object's header has already been restored.
+      return;
+    }
+    // The _header field is now marked. The winner's 'dmw' variable
+    // contains the original, unmarked header/dmw value and any
+    // losers have a marked header/dmw value that will be cleaned
+    // up below.
+  }
+
+  if (dmw->is_marked()) {
+    // Clear the mark from the header/dmw copy in preparation for
+    // possible restoration from this thread.
+    assert(dmw->hash() == 0, "hashcode must be 0: dmw=" INTPTR_FORMAT,
+           p2i(dmw));
+    dmw = dmw->set_unmarked();
+  }
+  assert(dmw->is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, p2i(dmw));
+
+  // Install displaced mark word if the object's header still points
+  // to this ObjectMonitor. All racing callers to this function will
+  // reach this point, but only one can win.
+  obj->cas_set_mark(dmw, markOopDesc::encode(this));
+
+  // Note: It does not matter which thread restored the header/dmw
+  // into the object's header. The thread deflating the monitor just
+  // wanted the object's header restored and it is. The threads that
+  // detected a race with the deflation process also wanted the
+  // object's header restored before they retry their operation and
+  // because it is restored they will only retry once.
+
+  if (marked_dmw != NULL) {
+    // Clear _header to NULL if it is still marked_dmw so a racing
+    // install_displaced_markword_in_object() can bail out sooner.
+    Atomic::cmpxchg((markOop)NULL, &_header, marked_dmw);
+  }
+}
+
 #define MAX_RECHECK_INTERVAL 1000
 
 void ObjectMonitor::EnterI(TRAPS) {
@@ -432,6 +522,21 @@
     return;
   }
 
+  if (_owner == DEFLATER_MARKER) {
+    // The deflation protocol finished the first part (setting _owner), but
+    // it failed the second part (making _contentions negative) and bailed.
+    // Because we're called from enter() we have at least one contention.
+    guarantee(_contentions > 0, "owner == DEFLATER_MARKER && contentions <= 0 "
+              "should have been handled by the caller: contentions=%d",
+              _contentions);
+    if (Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
+      // Acquired the monitor.
+      assert(_succ != Self, "invariant");
+      assert(_Responsible != Self, "invariant");
+      return;
+    }
+  }
+
   assert(InitDone, "Unexpectedly not initialized");
 
   // We try one round of spinning *before* enqueueing Self.
@@ -548,6 +653,19 @@
 
     if (TryLock(Self) > 0) break;
 
+    if (_owner == DEFLATER_MARKER) {
+      // The deflation protocol finished the first part (setting _owner), but
+      // it failed the second part (making _contentions negative) and bailed.
+      // Because we're called from enter() we have at least one contention.
+      guarantee(_contentions > 0, "owner == DEFLATER_MARKER && contentions <= 0 "
+                "should have been handled by the caller: contentions=%d",
+                _contentions);
+      if (Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
+        // Acquired the monitor.
+        break;
+      }
+    }
+
     // The lock is still contested.
     // Keep a tally of the # of futile wakeups.
     // Note that the counter is not protected by a lock or updated by atomics.
@@ -669,6 +787,19 @@
     if (TryLock(Self) > 0) break;
     if (TrySpin(Self) > 0) break;
 
+    if (_owner == DEFLATER_MARKER) {
+      // The deflation protocol finished the first part (setting _owner),
+      // but it will observe _waiters != 0 and will bail out. Because we're
+      // called from wait() we may or may not have any contentions.
+      guarantee(_contentions >= 0, "owner == DEFLATER_MARKER && contentions < 0 "
+                "should have been handled by the caller: contentions=%d",
+                _contentions);
+      if (Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
+        // Acquired the monitor.
+        break;
+      }
+    }
+
     // State transition wrappers around park() ...
     // ReenterI() wisely defers state transitions until
     // it's clear we must park the thread.
@@ -876,7 +1007,8 @@
       // way we should encounter this situation is in the presence of
       // unbalanced JNI locking. TODO: CheckJNICalls.
       // See also: CR4414101
-      assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
+      assert(false, "Non-balanced monitor enter/exit! Likely JNI locking: "
+             "owner=" INTPTR_FORMAT, p2i(_owner));
       return;
     }
   }
@@ -1126,16 +1258,20 @@
 
 // reenter() enters a lock and sets recursion count
 // complete_exit/reenter operate as a wait without waiting
-void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
+bool ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
   Thread * const Self = THREAD;
   assert(Self->is_Java_thread(), "Must be Java thread!");
   JavaThread *jt = (JavaThread *)THREAD;
 
   guarantee(_owner != Self, "reenter already owner");
-  enter(THREAD);       // enter the monitor
+  if (!enter(THREAD)) {
+    // Failed to enter the monitor so return for a retry.
+    return false;
+  }
+  // Entered the monitor.
   guarantee(_recursions == 0, "reenter recursion");
   _recursions = recursions;
-  return;
+  return true;
 }
 
 
@@ -1363,7 +1499,8 @@
     assert(_owner != Self, "invariant");
     ObjectWaiter::TStates v = node.TState;
     if (v == ObjectWaiter::TS_RUN) {
-      enter(Self);
+      const bool success = enter(Self);
+      ADIM_guarantee(success, "enter signaled for a retry, but monitor should not have been deflated as waiters > 0");
     } else {
       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
       ReenterI(Self, &node);
@@ -1926,3 +2063,76 @@
 
   DEBUG_ONLY(InitDone = true;)
 }
+
+// For internal used by ObjectSynchronizer::monitors_iterate().
+ObjectMonitorHandle::ObjectMonitorHandle(ObjectMonitor * om_ptr) {
+  om_ptr->inc_ref_count();
+  _om_ptr = om_ptr;
+}
+
+ObjectMonitorHandle::~ObjectMonitorHandle() {
+  if (_om_ptr != NULL) {
+    _om_ptr->dec_ref_count();
+    _om_ptr = NULL;
+  }
+}
+
+// Save the ObjectMonitor* associated with the specified markOop and
+// increment the ref_count. This function should only be called if
+// the caller has verified mark->has_monitor() == true. The object
+// parameter is needed to verify that ObjectMonitor* has not been
+// deflated and reused for another object.
+//
+// This function returns true if the ObjectMonitor* has been safely
+// saved. This function returns false if we have lost a race with
+// async deflation; the caller should retry as appropriate.
+//
+bool ObjectMonitorHandle::save_om_ptr(oop object, markOop mark) {
+  guarantee(mark->has_monitor(), "sanity check: mark=" INTPTR_FORMAT,
+            p2i(mark));
+
+  ObjectMonitor * om_ptr = mark->monitor();
+  om_ptr->inc_ref_count();
+
+  if (AsyncDeflateIdleMonitors) {
+    // Race here if monitor is not owned! The above ref_count bump
+    // will cause subsequent async deflation to skip it. However,
+    // previous or concurrent async deflation is a race.
+    if (om_ptr->_owner == DEFLATER_MARKER && om_ptr->_contentions <= 0) {
+      // Async deflation is in progress. Attempt to restore the
+      // header/dmw to the object's header so that we only retry once
+      // if the deflater thread happens to be slow.
+      om_ptr->install_displaced_markword_in_object(object);
+      om_ptr->dec_ref_count();
+      return false;
+    }
+    // The ObjectMonitor could have been deflated and reused for
+    // another object before we bumped the ref_count so make sure
+    // our object still refers to this ObjectMonitor.
+    const markOop tmp = object->mark();
+    if (!tmp->has_monitor() || tmp->monitor() != om_ptr) {
+      // Async deflation and reuse won the race so we have to retry.
+      // Skip object header restoration since that's already done.
+      om_ptr->dec_ref_count();
+      return false;
+    }
+  }
+
+  guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT,
+            p2i(_om_ptr));
+  _om_ptr = om_ptr;
+  return true;
+}
+
+// For internal use by ObjectSynchronizer::inflate().
+void ObjectMonitorHandle::set_om_ptr(ObjectMonitor * om_ptr) {
+  if (_om_ptr == NULL) {
+    guarantee(om_ptr != NULL, "cannot clear an unset om_ptr");
+    om_ptr->inc_ref_count();
+    _om_ptr = om_ptr;
+  } else {
+    guarantee(om_ptr == NULL, "can only clear a set om_ptr");
+    _om_ptr->dec_ref_count();
+    _om_ptr = NULL;
+  }
+}