8274338: com/sun/jdi/RedefineCrossEvent.java failed "assert(m != __null) failed: NULL mirror"

Reviewed-by: dholmes, iklam

src/hotspot/share/classfile/classLoaderData.cpp

@@ -353,6 +353,9 @@ void ClassLoaderData::methods_do(void f(Method*)) {
 }
 
 void ClassLoaderData::loaded_classes_do(KlassClosure* klass_closure) {
+  // To call this, one must have the MultiArray_lock held, but the _klasses list still has lock free reads.
+  assert_locked_or_safepoint(MultiArray_lock);
+
   // Lock-free access requires load_acquire
   for (Klass* k = Atomic::load_acquire(&_klasses); k != NULL; k = k->next_link()) {
     // Do not filter ArrayKlass oops here...

src/hotspot/share/oops/instanceKlass.cpp

@@ -2584,6 +2584,9 @@ void InstanceKlass::restore_unshareable_info(ClassLoaderData* loader_data, Handl
   constants()->restore_unshareable_info(CHECK);
 
   if (array_klasses() != NULL) {
+    // To get a consistent list of classes we need MultiArray_lock to ensure
+    // array classes aren't observed while they are being restored.
+     MutexLocker ml(MultiArray_lock);
     // Array classes have null protection domain.
     // --> see ArrayKlass::complete_create_array_klass()
     array_klasses()->restore_unshareable_info(ClassLoaderData::the_null_class_loader_data(), Handle(), CHECK);