unlock

    fn unlock(m: *ParkingMutex) void {
        state: switch (State.locked_once) { // assume 'locked_once' to optimize for uncontended case
            .unlocked => unreachable, // we hold the lock

            .locked_once => continue :state m.state.cmpxchgWeak(
                .locked_once,
                .unlocked,
                .release, // release lock
                .acquire, // acquire any `Waiter` memory
            ) orelse {
                @branchHint(.likely);
                return;
            },

            _ => |last_state| {
                // The logic here does not have ABA problems, and does some accesses non-atomically,
                // because `Waiter.next` is owned by the lock holder (that's us!) once the waiter is
                // in the linked list, up until we unpark the waiter.

                // Run through the waiter list to the end to ensure fairness. This is obviously not
                // ideal, but it shouldn't be a big deal in practice provided the critical section
                // is fairly small (so we won't get too many threads contending the mutex at once).
                // There's a *chance* we could get away with a LIFO queue for our use case, but I
                // don't wanna risk that.
                var parent: ?*Waiter = null;
                var waiter: *Waiter = last_state.waiter().?;
                while (waiter.next) |next| {
                    parent = waiter;
                    waiter = next;
                }
                // `waiter` is next in line for the lock. Remove them from the list.
                if (parent) |p| {
                    assert(p.next == waiter);
                    p.next = null;
                } else {
                    // We're waking the last waiter, so clear the list head.
                    if (m.state.cmpxchgWeak(
                        .fromWaiter(last_state.waiter().?),
                        .locked_once,
                        .acquire,
                        .acquire, // acquire any new `Waiter` memory
                    )) |new_state| {
                        continue :state new_state;
                    }
                }
                // Now we're ready to actually hand the lock over to them.
                const tid = waiter.tid; // load before the unpark below potentially invalidates `waiter`
                if (need_unpark_flag) setUnparkFlag(&waiter.unpark_flag);
                unpark(&.{tid}, m);
                return;
            },
        }
    }