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

[Vlastimil Babka <vbabka@suse.cz>]

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);