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

[Yury Norov <yury.norov@gmail.com>]

On Mon, Jun 30, 2025 at 07:55:49PM +0200, Jason A. Donenfeld wrote:
> 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) <yury.norov@gmail.com>
> > 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) <yury.norov@gmail.com>
> > ---
> >  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?

Sure, no problem.