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

Hi Yury,

> > > > > 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?
> > >
> > > It will return >= nd_cpu_ids. The original version based a for-loop
> > > does the same, so I decided that the caller is safe against it.
> >
> > Good point. I just checked... This goes into queue_work_on() which
> > eventually hits:
> >
> >         /* pwq which will be used unless @work is executing elsewhere */
> >         if (req_cpu == WORK_CPU_UNBOUND) {
> >
> > And it turns out WORK_CPU_UNBOUND is the same as nr_cpu_ids. So I guess
> > that's a fine failure mode.
>
> Actually, cpumask_nth_cpu may return >= nr_cpu_ids because of
> small_cpumask_nbits optimization. So it's safer to relax the
> condition.
>
> Can you consider applying the following patch for that?
>
> Thanks,
> Yury
>
>
> From fbdce972342437fb12703cae0c3a4f8f9e218a1b Mon Sep 17 00:00:00 2001
> From: Yury Norov (NVIDIA) <[email protected]>
> Date: Mon, 30 Jun 2025 13:47:49 -0400
> Subject: [PATCH] workqueue: relax condition in __queue_work()
>
> Some cpumask search functions may return a number greater than
> nr_cpu_ids when nothing is found. Adjust __queue_work() to it.
>
> Signed-off-by: Yury Norov (NVIDIA) <[email protected]>
> ---
>  kernel/workqueue.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 9f9148075828..abacfe157fe6 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2261,7 +2261,7 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
>         rcu_read_lock();
>  retry:
>         /* pwq which will be used unless @work is executing elsewhere */
> -       if (req_cpu == WORK_CPU_UNBOUND) {
> +       if (req_cpu >= WORK_CPU_UNBOUND) {
>                 if (wq->flags & WQ_UNBOUND)
>                         cpu = wq_select_unbound_cpu(raw_smp_processor_id());
>                 else
>

Seems reasonable to me... Maybe submit this to Tejun and CC me?

Jason