There's a way to make Arena::{new, drop} relaxed

[Avi-D-coder]

Currently they use a mutex, but the design calls for a Acquire Release CAS.
An ordering can be established, by with the traditional approach of guarding the Acquire read with a Relaxed read.
Each GC imperative will be sent with a epoch bit, the Relaxed drop message will contain the last read epoch bit. A ordering of the transferred memory range will only be established with a Release drop. This will be issued, when worker owns no Arenas of the previous epoch.

Note: A Release could issued on every drop, or at an even lower frequency. I will benchmark to determine the best approach, in theory it could even be sellected by profile guided optimization.