Re: [PATCH v2] wireguard: queueing: simplify wg_cpumask_next_online()

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Thu, Jun 19, 2025 at 10:54:59AM -0400, Yury Norov wrote:
> From: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> 
> wg_cpumask_choose_online() opencodes cpumask_nth(). Use it and make the
> function significantly simpler. While there, fix opencoded cpu_online()
> too.
> 
> Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> ---
> v1: https://lore.kernel.org/all/20250604233656.41896-1-yury.norov@gmail.com/
> v2:
>  - fix 'cpu' undeclared;
>  - change subject (Jason);
>  - keep the original function structure (Jason);
> 
>  drivers/net/wireguard/queueing.h | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/wireguard/queueing.h b/drivers/net/wireguard/queueing.h
> index 7eb76724b3ed..56314f98b6ba 100644
> --- a/drivers/net/wireguard/queueing.h
> +++ b/drivers/net/wireguard/queueing.h
> @@ -104,16 +104,11 @@ static inline void wg_reset_packet(struct sk_buff *skb, bool encapsulating)
>  
>  static inline int wg_cpumask_choose_online(int *stored_cpu, unsigned int id)
>  {
> -	unsigned int cpu = *stored_cpu, cpu_index, i;
> +	unsigned int cpu = *stored_cpu;
> +
> +	if (unlikely(cpu >= nr_cpu_ids || !cpu_online(cpu)))
> +		cpu = *stored_cpu = cpumask_nth(id % num_online_cpus(), cpu_online_mask);

I was about to apply this but then it occurred to me: what happens if
cpu_online_mask changes (shrinks) after num_online_cpus() is evaluated?
cpumask_nth() will then return nr_cpu_ids?

Jason