There's a small window when concurrent allocation requests can succeed (method_counters() == NULL check and set_method_counters() call) and leave unmanaged MethodCounters structures.

 http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/file/tip/src/share/vm/oops/method.cpp#l384

MethodCounters* Method::build_method_counters(Method* m, TRAPS) {
  methodHandle mh(m);
  ClassLoaderData* loader_data = mh->method_holder()->class_loader_data();
  MethodCounters* counters = MethodCounters::allocate(loader_data, CHECK_NULL);
  if (mh->method_counters() == NULL) {
    mh->set_method_counters(counters);
  } else {
    MetadataFactory::free_metadata(loader_data, counters);
  }
  return mh->method_counters();
}