See parent for related commentary, which I summarize below:-
The fix for 7028845 is a _strict_ subset of the much more extensive fixes for 6668573.
This (sub)CR was created expressly to backport an important and essential subset of 6668573
to hs20 (or earlier). The fix for 6668573 is already in hs21 and later, and includes the
changes in 7028845.
I've made a run of GC testlist with -XX:+UseConcMarkSweepGC -XX:+ParallelRefProcEnabled -XX:+DisableExplicitGC.