Reference Counting (including a secondary counter for internal cycles) is your silver bullet. The only hairy issue then becomes multiple processors needing to

Reference Counting (including a secondary counter for internal cycles) is your silver bullet. The only hairy issue then becomes multiple processors needing to update the reference counter, where the count has to go in an out of the various processor's cache. One possible way to resolve that is to have a per-thread reference count, and only change the globally shared reference count when a per-thread's reference count would reach 0.