Shenandoah keeps track of the gc state in `ShenandoahHeap::_gc_state`. This value is only changed on a safepoint. It is also propagated to (almost) all threads on a safepoint. Apart from verification, changing this value is the only reason for `init-update-refs` safepoint to exist. If we can manage `_gc_state` is a purely thread local variable, we could eliminate this safepoint.