Re: [PATCH v2] mm/slub: mark racy accesses on slab->slabs

Vlastimil Babka
2024-03-25T08:48:10Z

On 3/21/24 4:48 AM, linke li wrote:
> The reads of slab->slabs are racy because it may be changed by
> put_cpu_partial concurrently. In slabs_cpu_partial_show() and
> show_slab_objects(), slab->slabs is only used for showing information.
>
> Data-racy reads from shared variables that are used only for diagnostic
> purposes should typically use data_race(), since it is normally not a
> problem if the values are off by a little.
>
> This patch is aimed at reducing the number of benign races reported by
> KCSAN in order to focus future debugging effort on harmful races.
>
> Signed-off-by: linke li <lilinke99@qq.com>
> Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>

Chengming provided feedback to v1 but not offered a Reviewed-by: AFAICS? Or
maybe will offer it now? :)

Vlastimil

> ---
> mm/slub.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 2ef88bbf56a3..0d700f6ca547 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6052,7 +6052,7 @@ static ssize_t show_slab_objects(struct kmem_cache *s,
> else if (flags & SO_OBJECTS)
> WARN_ON_ONCE(1);
> else
> - x = slab->slabs;
> + x = data_race(slab->slabs);
> total += x;
> nodes[node] += x;
> }
> @@ -6257,7 +6257,7 @@ static ssize_t slabs_cpu_partial_show(struct kmem_cache *s, char *buf)
> slab = slub_percpu_partial(per_cpu_ptr(s->cpu_slab, cpu));
>
> if (slab)
> - slabs += slab->slabs;
> + slabs += data_race(slab->slabs);
> }
> #endif
>
> @@ -6271,7 +6271,7 @@ static ssize_t slabs_cpu_partial_show(struct kmem_cache *s, char *buf)
>
> slab = slub_percpu_partial(per_cpu_ptr(s->cpu_slab, cpu));
> if (slab) {
> - slabs = READ_ONCE(slab->slabs);
> + slabs = data_race(slab->slabs);
> objects = (slabs * oo_objects(s->oo)) / 2;
> len += sysfs_emit_at(buf, len, " C%d=%d(%d)",
> cpu, objects, slabs);