lock

    pub fn lock(m: *Mutex, io: Io) Cancelable!void {
        const initial_state = m.state.cmpxchgStrong(
            .unlocked,
            .locked_once,
            .acquire,
            .monotonic,
        ) orelse {
            @branchHint(.likely);
            return;
        };
        if (initial_state == .contended) {
            try io.futexWait(State, &m.state.raw, .contended);
        }
        while (m.state.swap(.contended, .acquire) != .unlocked) {
            try io.futexWait(State, &m.state.raw, .contended);
        }
    }