Atomics and Ordering

Start with the idea

Ordering is the interesting half of the answer. Relaxed guarantees only that the operation itself is indivisible; Acquire and Release pair up to publish everything written before a release to whoever performs the matching acquire; SeqCst adds a single global order and costs the most. SeqCst provides a stronger model, but it does not repair a flawed protocol and its cost depends on the architecture and operation.

An atomic operation is indivisible with respect to competing atomic accesses, but a sequence of two atomics is not automatically a transaction. For a shared event counter whose final value is read after all workers join, fetch_add with Relaxed ordering may be enough: the count is accurate, and the join supplies the completion boundary.