GenCollectedHeap::collect_generation() does some seemingly collector specific changes to discovery in generic code. This seems to be for CMS.

There is also the comment:
    // Note on ref discovery: For what appear to be historical reasons,
    // GCH enables and disabled (by enqueing) refs discovery.
    // In the future this should be moved into the generation's
    // collect method so that ref discovery and enqueueing concerns
    // are local to a generation. The collect method could return
    // an appropriate indication in the case that notification on
    // the ref lock was needed. This will make the treatment of
    // weak refs more uniform (and indeed remove such concerns
    // from GCH). XXX

Do as the comment suggest and move discovery control into the collector specific collect() implementations.