8274338: com/sun/jdi/RedefineCrossEvent.java failed "assert(m != __nu…

…ll) failed: NULL mirror"

Reviewed-by: phh
Backport-of: 172aed1a2d75756b140cb723133ac5fb67f7745e

src/hotspot/share/oops/instanceKlass.cpp

@@ -2367,6 +2367,9 @@ 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);