Re: [PATCH] workqueue: validate cpumask_first() result in llc_populate_cpu_shard_id()

[Breno Leitao <leitao@debian.org>]

On Fri, Apr 10, 2026 at 07:36:10AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Fri, Apr 10, 2026 at 03:42:00AM -0700, Breno Leitao wrote:
> > @@ -8318,7 +8319,17 @@ static void __init llc_populate_cpu_shard_id(const struct cpumask *pod_cpus,
> >                          * The siblings' shard MUST be the same as the leader.
> >                          * never split threads in the same core.
> >                          */
> > -                       cpu_shard_id[c] = cpu_shard_id[cpumask_first(sibling_cpus)];
> > +                       leader = cpumask_first(sibling_cpus);
> > +
> > +                       /*
> > +                        * sibling_cpus cannot be empty here since 'c'
> > +                        * is always set in it. This check silences a
> > +                        * compiler warning about using the unchecked
> > +                        * cpumask_first() result as an array index.
> > +                        */
> 
> Can you add more details on the warning and update the patch desc
> accordingly?

Ack!

> This only triggers on UP configs, right?

yes, this only triggers on UP configs (like nios2) because on SMP
NR_CPUS > 1 and the compiler can't prove the index is out of bounds.