kernel/workqueue.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
In llc_populate_cpu_shard_id(), cpumask_first(sibling_cpus) is used to
find the leader CPU, and the result is then used to index into
cpu_shard_id[]. Add a bounds check with WARN_ON_ONCE to guard against
unexpected values before using it as an array index.
Store the result in a local variable to make the code clearer, as also
to avoid calling cpumask_first() twice.
Fixes: 5920d046f7ae3 ("workqueue: add WQ_AFFN_CACHE_SHARD affinity scope")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202604022343.GQtkF2vO-lkp@intel.com/
Signed-off-by: Breno Leitao <leitao@debian.org>
---
kernel/workqueue.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 083d8fe301f46..3cb4376a248b0 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -8300,6 +8300,7 @@ static void __init llc_populate_cpu_shard_id(const struct cpumask *pod_cpus,
int cores_in_shard = 0;
/* This is a cursor for the shards. Go from zero to nr_shards - 1*/
int shard_id = 0;
+ int leader;
int c;
/* Iterate at every CPU for a given LLC pod, and assign it a shard */
@@ -8318,7 +8319,11 @@ 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);
+
+ if (WARN_ON_ONCE(leader >= nr_cpu_ids))
+ continue;
+ cpu_shard_id[c] = cpu_shard_id[leader];
}
}
---
base-commit: 3fa7d958829eb9bc3b469ed07f11de3d2804ef71
change-id: 20260410-workqueue_fix_nios-e6763904aee9
Best regards,
--
Breno Leitao <leitao@debian.org>Hello,
On Fri, Apr 10, 2026 at 01:49:50AM -0700, Breno Leitao wrote:
> In llc_populate_cpu_shard_id(), cpumask_first(sibling_cpus) is used to
> find the leader CPU, and the result is then used to index into
> cpu_shard_id[]. Add a bounds check with WARN_ON_ONCE to guard against
> unexpected values before using it as an array index.
>
> Store the result in a local variable to make the code clearer, as also
> to avoid calling cpumask_first() twice.
>
> Fixes: 5920d046f7ae3 ("workqueue: add WQ_AFFN_CACHE_SHARD affinity scope")
...
> @@ -8318,7 +8319,11 @@ 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);
> +
> + if (WARN_ON_ONCE(leader >= nr_cpu_ids))
> + continue;
> + cpu_shard_id[c] = cpu_shard_id[leader];
sibling_cpus can't be empty, right? This is mostly to shut up the reported
compiler warning? If so, can you please note that in a ocmment and the
description?
Thanks.
--
tejun
Hello Tejun,
On Thu, Apr 09, 2026 at 11:00:30PM -1000, Tejun Heo wrote:
> On Fri, Apr 10, 2026 at 01:49:50AM -0700, Breno Leitao wrote:
> > In llc_populate_cpu_shard_id(), cpumask_first(sibling_cpus) is used to
> > find the leader CPU, and the result is then used to index into
> > cpu_shard_id[]. Add a bounds check with WARN_ON_ONCE to guard against
> > unexpected values before using it as an array index.
> >
> > Store the result in a local variable to make the code clearer, as also
> > to avoid calling cpumask_first() twice.
> >
> > Fixes: 5920d046f7ae3 ("workqueue: add WQ_AFFN_CACHE_SHARD affinity scope")
> ...
> > @@ -8318,7 +8319,11 @@ 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);
> > +
> > + if (WARN_ON_ONCE(leader >= nr_cpu_ids))
> > + continue;
> > + cpu_shard_id[c] = cpu_shard_id[leader];
>
> sibling_cpus can't be empty, right?
Correct. sibling_cpus will have, at least, 'c' set.
> This is mostly to shut up the reported
> compiler warning? If so, can you please note that in a ocmment and the
> description?
Sure. Is something like the following acceptable?
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 083d8fe301f46..5dc304cdfa7f9 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -8300,6 +8300,7 @@ static void __init llc_populate_cpu_shard_id(const struct cpumask *pod_cpus,
int cores_in_shard = 0;
/* This is a cursor for the shards. Go from zero to nr_shards - 1*/
int shard_id = 0;
+ int leader;
int c;
/* Iterate at every CPU for a given LLC pod, and assign it a shard */
@@ -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.
+ */
+ if (WARN_ON_ONCE(leader >= nr_cpu_ids))
+ continue;
+ cpu_shard_id[c] = cpu_shard_id[leader];
}
}
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? This only triggers on UP configs, right? Thanks. -- tejun
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.
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