> If you reference N twice and M zero times, why did you define M? Did you make a typo in the O() or is this intentional?

That was a typo. It should have read `O(N log(M))`.

> Does getting a new value automatically release a reference to the old one (so it's no longer safe to read)?

Correct. Every read (get) of the variable causes the previous value to no longer be safe to use in that thread.