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	Mon, 13 Apr 2026 14:21:18 +0000
Date: Mon, 13 Apr 2026 07:21:14 -0700
From: Breno Leitao <leitao@debian.org>
To: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>, linux-kernel@vger.kernel.org, 
	kernel-team@meta.com, kernel test robot <lkp@intel.com>
Subject: Re: [PATCH] workqueue: validate cpumask_first() result in
 llc_populate_cpu_shard_id()
Message-ID: <adz4IVO4Ury-bu-S@gmail.com>
References: <20260410-workqueue_fix_nios-v1-1-abbb26575b1b@debian.org>
 <adi8Llt3tW-EwpPt@slm.duckdns.org>
 <adjTrRL-PrCRUd57@gmail.com>
 <adk1CtVuElIw5bxo@slm.duckdns.org>
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In-Reply-To: <adk1CtVuElIw5bxo@slm.duckdns.org>
X-Debian-User: leitao

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.