I don't know what you mean here: "than copy the map while spinning on an atomic." Why would we copy the map? And by "spinning on an atomic", do you mean just spin-waiting on some flag? That's, morally, a lock. A well designed lock free algorithm allows all threads to make progress at the same time, and if there is contention, an alternative that hopefully is not just redoing that same work. The cost is more atomic operations, which means that if there is low contention, you're paying more overhead than needed.