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

["David Hildenbrand (Arm)" <david@kernel.org>]

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);
 }

But I'll let others decide :)

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David