diff -r 8b6cc0bb93d0 -r 9186be5c78ba src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp	Wed Nov 27 06:36:41 2019 -0800
+++ b/src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp	Wed Nov 27 11:52:57 2019 -0500
@@ -32,6 +32,7 @@
 #include "gc/shenandoah/shenandoahLock.hpp"
 #include "gc/shenandoah/shenandoahEvacOOMHandler.hpp"
 #include "gc/shenandoah/shenandoahSharedVariables.hpp"
+#include "gc/shenandoah/shenandoahUnload.hpp"
 #include "services/memoryManager.hpp"
 
 class ConcurrentGCTimer;
@@ -271,6 +272,7 @@
   ShenandoahSharedFlag   _full_gc_in_progress;
   ShenandoahSharedFlag   _full_gc_move_in_progress;
   ShenandoahSharedFlag   _progress_last_gc;
+  ShenandoahSharedFlag   _concurrent_root_in_progress;
 
   void set_gc_state_all_threads(char state);
   void set_gc_state_mask(uint mask, bool value);
@@ -287,6 +289,7 @@
   void set_full_gc_move_in_progress(bool in_progress);
   void set_concurrent_traversal_in_progress(bool in_progress);
   void set_has_forwarded_objects(bool cond);
+  void set_concurrent_root_in_progress(bool cond);
 
   inline bool is_stable() const;
   inline bool is_idle() const;
@@ -299,6 +302,8 @@
   inline bool is_concurrent_traversal_in_progress() const;
   inline bool has_forwarded_objects() const;
   inline bool is_gc_in_progress_mask(uint mask) const;
+  inline bool is_stw_gc_in_progress() const;
+  inline bool is_concurrent_root_in_progress() const;
 
 // ---------- GC cancellation and degeneration machinery
 //
@@ -511,6 +516,7 @@
 //
 private:
   ShenandoahSharedFlag _unload_classes;
+  ShenandoahUnload     _unloader;
 
 public:
   void set_unload_classes(bool uc);
@@ -523,6 +529,12 @@
   void stw_unload_classes(bool full_gc);
   void stw_process_weak_roots(bool full_gc);
 
+  // Prepare concurrent root processing
+  void prepare_concurrent_roots();
+  // Prepare and finish concurrent unloading
+  void prepare_concurrent_unloading();
+  void finish_concurrent_unloading();
+
 // ---------- Generic interface hooks
 // Minor things that super-interface expects us to implement to play nice with
 // the rest of runtime. Some of the things here are not required to be implemented,
@@ -562,7 +574,7 @@
 public:
   void register_nmethod(nmethod* nm);
   void unregister_nmethod(nmethod* nm);
-  void flush_nmethod(nmethod* nm) {}
+  void flush_nmethod(nmethod* nm);
   void verify_nmethod(nmethod* nm) {}
 
 // ---------- Pinning hooks