The UseMembar flag was deprecated in JDK 10, and is default true/enabled on all platforms since that same release. It should be obsoleted and the then dead serialization page functionality it controls should be removed.
JDK-8152292Consider using proper OS APIs for os::serialize_thread_states
JDK-8143878Memory serialization page can become a bottleneck
JDK-8222231Clean up interfaceSupport.inline.hpp duplicated code
JDK-8187809UseMembar should be set true and deprecate the flag