“Atomically sees the old value” means you’re doing an atomic RMW, which is more expensive than just a store with a release fence. Atomic RMW is almost as expen

“Atomically sees the old value” means you’re doing an atomic RMW, which is more expensive than just a store with a release fence. Atomic RMW is almost as expensive as CAS. Exactly as expensive as CAS on some cpus.