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

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

Posted by linke li 1 year, 10 months ago

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>
---
 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);
-- 
2.39.3 (Apple Git-146)

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

Posted by Vlastimil Babka 1 year, 10 months ago

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);

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

Posted by Chengming Zhou 1 year, 10 months ago

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@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? :)

Ah, right.

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

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);
>

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

Posted by Vlastimil Babka 1 year, 10 months ago

On 3/25/24 9:49 AM, Chengming Zhou wrote:
> 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@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? :)
> 
> Ah, right.
> 
> Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>

Thanks, added to slab/for-6.10/cleanup