src/share/vm/code/nmethod.hpp

@@ -192,11 +192,14 @@
 
   jbyte _scavenge_root_state;
 
   NOT_PRODUCT(bool _has_debug_info; )
 
-  // Nmethod Flushing lock (if non-zero, then the nmethod is not removed)
+  // Nmethod Flushing lock. If non-zero, then the nmethod is not removed
+  // and is not made into a zombie. However, once the nmethod is made into
+  // a zombie, it will be locked one final time if CompiledMethodUnload
+  // event processing needs to be done.
   jint  _lock_count;
 
   // not_entrant method removal. Each mark_sweep pass will update
   // this mark to current sweep invocation count if it is seen on the
   // stack.  An not_entrant method can be removed when there is no

@@ -520,12 +523,13 @@
 
  protected:
   void flush();
 
  public:
-  // If returning true, it is unsafe to remove this nmethod even though it is a zombie
-  // nmethod, since the VM might have a reference to it. Should only be called from a  safepoint.
+  // When true is returned, it is unsafe to remove this nmethod even if
+  // it is a zombie, since the VM or the ServiceThread might still be
+  // using it. Should only be called from a safepoint.
   bool is_locked_by_vm() const                    { return _lock_count >0; }
 
   // See comment at definition of _last_seen_on_stack
   void mark_as_seen_on_stack();
   bool can_not_entrant_be_converted();

@@ -687,17 +691,24 @@
   static int osr_entry_point_offset()             { return offset_of(nmethod, _osr_entry_point); }
   static int entry_bci_offset()                   { return offset_of(nmethod, _entry_bci); }
 
 };
 
-// Locks an nmethod so its code will not get removed, even if it is a zombie/not_entrant method
+// Locks an nmethod so its code will not get removed and it will not
+// be made into a zombie, even if it is a not_entrant method. After the
+// nmethod becomes a zombie, if CompiledMethodUnload event processing
+// needs to be done, then lock_nmethod() is used directly to keep the
+// generated code from being reused too early.
 class nmethodLocker : public StackObj {
   nmethod* _nm;
 
  public:
 
-  static void lock_nmethod(nmethod* nm);   // note: nm can be NULL
+  // note: nm can be NULL
+  // Only JvmtiDeferredEvent::compiled_method_unload_event()
+  // should pass zombie_ok == true.
+  static void lock_nmethod(nmethod* nm, bool zombie_ok = false);
   static void unlock_nmethod(nmethod* nm); // (ditto)
 
   nmethodLocker(address pc); // derive nm from pc
   nmethodLocker(nmethod *nm) { _nm = nm; lock_nmethod(_nm); }
   nmethodLocker() { _nm = NULL; }