Terminology opinion: I'm not too fond of the term callback for "synchronous callback" as described in the post. First, it doesn't actually "call back", and secondly why not just use "function"?
Could also add "continuation" to the list perhaps, from continuation passing style (CPS). But that might not be a callback, as it never really goes back, but always goes forward.
It's all pretty much the same.
I like "synchronous callback" because this tells you two things:
1. it's synchronous, no need to add memory barriers
2. it _is_ a callback, it is called after the fact.