Re: [PATCH v3] mm: annotate data-race in cpu_needs_drain()

[Lorenzo Stoakes <ljs@kernel.org>]

On Fri, Jun 26, 2026 at 10:17:14AM +0200, David Hildenbrand (Arm) wrote:
> On 6/26/26 07:37, Xuewen Wang wrote:
> > KCSAN reports a data-race when cpu_needs_drain() reads another CPU's
> > per-cpu folio_batch->nr without locking, while the owning CPU writes
> > to it via folio_batch_add().
> >
> > Reading a slightly stale value is harmless -- cpu_needs_drain() only
> > decides whether to schedule a drain, and the next iteration of
> > __lru_add_drain_all() will re-check. Use data_race() to annotate
> > the intentional race.
> >
> > Signed-off-by: Xuewen Wang <wangxuewen@kylinos.cn>
> > ---
> > Changes in v3:
> > - Wrap the entire || expression in a single data_race() instead of wrapping
> >   each folio_batch_count() call individually, as suggested by Pedro and Lorenzo.
> >   This is equally effective and more readable.
> > - Remove data_race() from need_mlock_drain(), as it is now covered by the data_race()
> >   in cpu_needs_drain().
> > v2:
> >   https://lore.kernel.org/all/20260625065153.1581419-1-wangxuewen@kylinos.cn/
> > v1:
> >   https://lore.kernel.org/all/20260624092606.1083449-1-wangxuewen@kylinos.cn/
> > ---
> >  mm/swap.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/swap.c b/mm/swap.c
> > index 588f50d8f1a8..46ea207e0624 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -828,13 +828,13 @@ static bool cpu_needs_drain(unsigned int cpu)
> >  	struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu);
> >
> >  	/* Check these in order of likelihood that they're not zero */
> > -	return folio_batch_count(&fbatches->lru_add) ||
> > +	return data_race(folio_batch_count(&fbatches->lru_add) ||
> >  		folio_batch_count(&fbatches->lru_move_tail) ||
> >  		folio_batch_count(&fbatches->lru_deactivate_file) ||
> >  		folio_batch_count(&fbatches->lru_deactivate) ||
> >  		folio_batch_count(&fbatches->lru_lazyfree) ||
> >  		folio_batch_count(&fbatches->lru_activate) ||
> > -		need_mlock_drain(cpu) ||
> > +		need_mlock_drain(cpu)) ||
>
> The indentation is a bit suboptimal now.
>
> Would read nicer as
>
> diff --git a/mm/swap.c b/mm/swap.c
> index 588f50d8f1a8c..5958e6fdd3593 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -828,13 +828,13 @@ static bool cpu_needs_drain(unsigned int cpu)
>         struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu);
>
>         /* Check these in order of likelihood that they're not zero */
> -       return folio_batch_count(&fbatches->lru_add) ||
> -               folio_batch_count(&fbatches->lru_move_tail) ||
> -               folio_batch_count(&fbatches->lru_deactivate_file) ||
> -               folio_batch_count(&fbatches->lru_deactivate) ||
> -               folio_batch_count(&fbatches->lru_lazyfree) ||
> -               folio_batch_count(&fbatches->lru_activate) ||
> -               need_mlock_drain(cpu) ||
> +       return data_race(folio_batch_count(&fbatches->lru_add) ||
> +                        folio_batch_count(&fbatches->lru_move_tail) ||
> +                        folio_batch_count(&fbatches->lru_deactivate_file) ||
> +                        folio_batch_count(&fbatches->lru_deactivate) ||
> +                        folio_batch_count(&fbatches->lru_lazyfree) ||
> +                        folio_batch_count(&fbatches->lru_activate) ||
> +                        need_mlock_drain(cpu)) ||
>                 has_bh_in_lru(cpu, NULL);

Yeah that works for me.

Andrew - maybe easier if you fix that up? :)

>  }
>
> But I'll let others decide :)
>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
>
> --
> Cheers,
>
> David

Cheers, Lorenzo