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

[Chengming Zhou]

On 2024/3/25 16:48, Vlastimil Babka wrote:
> 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@xxxxxx>
>> Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>
> 
> Chengming provided feedback to v1 but not offered a Reviewed-by: AFAICS? Or
> maybe will offer it now? :)

Ah, right.

Reviewed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>

Thanks.

> 
> 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);
>