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

On Mon, Jun 30, 2025 at 01:33:37PM -0400, Yury Norov wrote:
> 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] <[email protected]>
> > > 
> > > 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] <[email protected]>
> > > ---
> > > v1: https://lore.kernel.org/all/[email protected]/
> > > 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.

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.

I'll queue this patch up.

Jason