add

        fn add(context: ?*anyopaque) callconv(.c) void {
            const waiter: *Waiter = @ptrCast(@alignCast(context));
            const futex = waiter.futex;
            _ = @atomicRmw(usize, &futex.num_waiters, .Add, 1, .acquire);
            waiter.tryAdd() catch |err| switch (err) {
                error.CancelRequested => {
                    wake(waiter);
                    assert(@atomicRmw(usize, &futex.num_waiters, .Sub, 1, .monotonic) >= 1);
                },
            };
        }