diff --git a/DONT_MERGE b/DONT_MERGE
new file mode 100644
index 00000000..e69de29b
diff --git a/openjdk/mmtkUpcalls.cpp b/openjdk/mmtkUpcalls.cpp
index fd90cf0e..c665a191 100644
--- a/openjdk/mmtkUpcalls.cpp
+++ b/openjdk/mmtkUpcalls.cpp
@@ -87,6 +87,15 @@ static void mmtk_resume_mutators(void *tls) {
     MutexLockerEx locker(MMTkHeap::heap()->gc_lock(), Mutex::_no_safepoint_check_flag);
     MMTkHeap::heap()->gc_lock()->notify_all();
   }
+
+  log_debug(gc)("Notifying mutators blocking on Heap_lock for reference pending list...");
+  // Note: That's the ReferenceHandler thread.
+  {
+    MutexLockerEx x(Heap_lock, Mutex::_no_safepoint_check_flag);
+    if (Universe::has_reference_pending_list()) {
+      Heap_lock->notify_all();
+    }
+  }
 }
 
 static const int GC_THREAD_KIND_WORKER = 1;
@@ -293,6 +302,8 @@ static void mmtk_enqueue_references(void** objects, size_t len) {
     return;
   }
 
+  MutexLocker x(Heap_lock);
+
   oop first = (oop) objects[0]; // This points to the first node of the linked list.
   oop last = first; // This points to the last node of the linked list.
 
@@ -313,6 +324,7 @@ static void mmtk_enqueue_references(void** objects, size_t len) {
 
   oop old_first = Universe::swap_reference_pending_list(first);
   HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(last, java_lang_ref_Reference::discovered_offset, old_first);
+  assert(Universe::has_reference_pending_list(), "Reference pending list is empty after swap");
 }
 
 OpenJDK_Upcalls mmtk_upcalls = {
diff --git a/openjdk/mmtkVMOperation.cpp b/openjdk/mmtkVMOperation.cpp
index 7ba6053f..51227ee3 100644
--- a/openjdk/mmtkVMOperation.cpp
+++ b/openjdk/mmtkVMOperation.cpp
@@ -32,21 +32,8 @@ VM_MMTkSTWOperation::VM_MMTkSTWOperation(MMTkVMCompanionThread *companion_thread
     _companion_thread(companion_thread) {
 }
 
-bool VM_MMTkSTWOperation::doit_prologue() {
-    Heap_lock->lock();
-    return true;
-}
-
 void VM_MMTkSTWOperation::doit() {
     log_trace(vmthread)("Entered VM_MMTkSTWOperation::doit().");
     _companion_thread->do_mmtk_stw_operation();
     log_trace(vmthread)("Leaving VM_MMTkSTWOperation::doit()");
 }
-
-void VM_MMTkSTWOperation::doit_epilogue() {
-    // Notify the reference processing thread
-    if (Universe::has_reference_pending_list()) {
-        Heap_lock->notify_all();
-    }
-    Heap_lock->unlock();
-}
diff --git a/openjdk/mmtkVMOperation.hpp b/openjdk/mmtkVMOperation.hpp
index 49897ce7..98d37ea4 100644
--- a/openjdk/mmtkVMOperation.hpp
+++ b/openjdk/mmtkVMOperation.hpp
@@ -39,9 +39,7 @@ class VM_MMTkSTWOperation : public VM_MMTkOperation {
 
 public:
   VM_MMTkSTWOperation(MMTkVMCompanionThread *companion_thread);
-  virtual bool doit_prologue() override;
   virtual void doit() override;
-  virtual void doit_epilogue() override;
 };
 
 #endif // MMTK_OPENJDK_MMTK_VM_OPERATION_HPP