Today we call SystemDictionaryShared::check_excluded_classes() before holding the DumpTimeTable_lock:

https://github.com/openjdk/jdk/blob/0b09129faefb945c2f2c35dab2731961216d054e/src/hotspot/share/cds/metaspaceShared.cpp#L490

  SystemDictionaryShared::check_excluded_classes();
  MutexLocker ml(DumpTimeTable_lock, Mutex::_no_safepoint_check_flag);

It's possible for concurrent class unloading to modify the dumptime table where SystemDictionaryShared::remove_dumptime_info is called from here:

 0 InstanceKlass::unload_class
 1 ClassLoaderData::classes_do
 2 ClassLoaderData::unload
 3 ClassLoaderDataGraph::do_unloading
 4 SystemDictionary::do_unloading
 5 ZUnload::unlink
 6 ZHeap::process_non_strong_references
 7 ZDriver::concurrent_process_non_strong_references
 8 ZDriver::gc
 9 ZDriver::run_service
10 ConcurrentGCThread::run

Fix: the above two source lines should be swapped.