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

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

On Mon, Jun 30, 2025 at 07:24:33PM +0200, Jason A. Donenfeld wrote:
> 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?

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.

If not, I can send a v3. But, what should we do - retry, or return a
local cpu? Or something else?

Thanks,
Yury