8047812: Ensure ClassLoaderDataGraph::classes_unloading_do only delivers...

8047812: Ensure ClassLoaderDataGraph::classes_unloading_do only delivers klasses from CLDs with non-reclaimed class loader oops
Reviewed-by: coleenp, sspitsyn, ehelin

src/share/vm/classfile/classLoaderData.cpp

@@ -531,6 +531,7 @@ bool ClassLoaderData::contains_klass(Klass* klass) {
 // GC root of class loader data created.
 ClassLoaderData* ClassLoaderDataGraph::_head = NULL;
 ClassLoaderData* ClassLoaderDataGraph::_unloading = NULL;
+ClassLoaderData* ClassLoaderDataGraph::_saved_unloading = NULL;
 ClassLoaderData* ClassLoaderDataGraph::_saved_head = NULL;
 
 bool ClassLoaderDataGraph::_should_purge = false;
@@ -628,7 +629,9 @@ void ClassLoaderDataGraph::loaded_classes_do(KlassClosure* klass_closure) {
 
 void ClassLoaderDataGraph::classes_unloading_do(void f(Klass* const)) {
   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint!");
-  for (ClassLoaderData* cld = _unloading; cld != NULL; cld = cld->next()) {
+  // Only walk the head until any clds not purged from prior unloading
+  // (CMS doesn't purge right away).
+  for (ClassLoaderData* cld = _unloading; cld != _saved_unloading; cld = cld->next()) {
     cld->classes_do(f);
   }
 }
@@ -676,6 +679,11 @@ bool ClassLoaderDataGraph::do_unloading(BoolObjectClosure* is_alive_closure) {
   ClassLoaderData* data = _head;
   ClassLoaderData* prev = NULL;
   bool seen_dead_loader = false;
+
+  // Save previous _unloading pointer for CMS which may add to unloading list before
+  // purging and we don't want to rewalk the previously unloaded class loader data.
+  _saved_unloading = _unloading;
+
   // mark metadata seen on the stack and code cache so we can delete
   // unneeded entries.
   bool has_redefined_a_class = JvmtiExport::has_redefined_a_class();

src/share/vm/classfile/classLoaderData.hpp

@@ -66,6 +66,7 @@ class ClassLoaderDataGraph : public AllStatic {
   static ClassLoaderData* _unloading;
   // CMS support.
   static ClassLoaderData* _saved_head;
+  static ClassLoaderData* _saved_unloading;
   static bool _should_purge;
 
   static ClassLoaderData* add(Handle class_loader, bool anonymous, TRAPS);