8268778: CDS check_excluded_classes needs DumpTimeTable_lock

Reviewed-by: ccheung, minqi

src/hotspot/share/cds/dynamicArchive.cpp

@@ -107,9 +107,11 @@ public:
 
     verify_universe("Before CDS dynamic dump");
     DEBUG_ONLY(SystemDictionaryShared::NoClassLoadingMark nclm);
+
+    // Block concurrent class unloading from changing the _dumptime_table
+    MutexLocker ml(DumpTimeTable_lock, Mutex::_no_safepoint_check_flag);
     SystemDictionaryShared::check_excluded_classes();
 
-    MutexLocker ml(DumpTimeTable_lock, Mutex::_no_safepoint_check_flag);
     init_header();
     gather_source_objs();
     reserve_buffer();

src/hotspot/share/cds/metaspaceShared.cpp

@@ -484,12 +484,10 @@ void VM_PopulateDumpSharedSpace::doit() {
 
   NOT_PRODUCT(SystemDictionary::verify();)
 
-  // At this point, many classes have been loaded.
+  // Block concurrent class unloading from changing the _dumptime_table
-  // Gather systemDictionary classes in a global array and do everything to
+  MutexLocker ml(DumpTimeTable_lock, Mutex::_no_safepoint_check_flag);
-  // that so we don't have to walk the SystemDictionary again.
   SystemDictionaryShared::check_excluded_classes();
 
-  MutexLocker ml(DumpTimeTable_lock, Mutex::_no_safepoint_check_flag);
   StaticArchiveBuilder builder;
   builder.gather_source_objs();
   builder.reserve_buffer();

src/hotspot/share/classfile/systemDictionaryShared.cpp

@@ -1485,6 +1485,8 @@ public:
 };
 
 void SystemDictionaryShared::check_excluded_classes() {
+  assert(no_class_loading_should_happen(), "sanity");
+  assert_lock_strong(DumpTimeTable_lock);
   ExcludeDumpTimeSharedClasses excl;
   _dumptime_table->iterate(&excl);
   _dumptime_table->update_counts();