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

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

[linke li]

The reads of slab->slabs are racy because it may be changed by 
put_cpu_partial concurrently. And in slabs_cpu_partial_show ->slabs is
only used for output. 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>
---
 mm/slub.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 2ef88bbf56a3..7b20591e7f8a 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -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] mm/slub: mark racy accesses on slab->slabs

[Chengming Zhou]

On 2024/3/9 15:48, linke li wrote:
> The reads of slab->slabs are racy because it may be changed by 
> put_cpu_partial concurrently. And in slabs_cpu_partial_show ->slabs is
> only used for output. 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>
> ---
>  mm/slub.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 2ef88bbf56a3..7b20591e7f8a 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -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);

There is another unmarked access of "slab->slabs" in the show_slab_objects(),
which you can change too.

I'm not sure that it's really safe to access "slab->slabs" here without any protection?
Although it should be no problem in practice, alternative choice maybe putting partial
slabs count in the kmem_cache_cpu struct.

Thanks.

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

[linke li]

Sorry for a late reply, I just found this because of my bad email client.

> There is another unmarked access of "slab->slabs" in the show_slab_objects(),
> which you can change too.

Yes, I think show_slab_objects() has a similar situation. Should I
consider to submit a V2 patch for this?

> I'm not sure that it's really safe to access "slab->slabs" here without any protection?
> Although it should be no problem in practice, alternative choice maybe putting partial
> slabs count in the kmem_cache_cpu struct.

I think it is ok, because it seems that slab->slabs in slub_percpu_partial
and show_slab_objects() are just used for showing some infomation.

I noticed Paul summarized some of these strategies in access-marking.txt[1]

Quote from it:

"Use of the data_race() Macro
----------------------------

Here are some situations where data_race() should be used instead of
READ_ONCE() and WRITE_ONCE():

1.	Data-racy loads from shared variables whose values are used only
	for diagnostic purposes.

2.	Data-racy reads whose values are checked against marked reload.

3.	Reads whose values feed into error-tolerant heuristics.

4.	Writes setting values that feed into error-tolerant heuristics.
"

Thanks,
Linke

[1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt

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

[Chengming Zhou]

On 2024/3/21 10:48, linke li wrote:
> Sorry for a late reply, I just found this because of my bad email client.
> 
>> There is another unmarked access of "slab->slabs" in the show_slab_objects(),
>> which you can change too.
> 
> Yes, I think show_slab_objects() has a similar situation. Should I
> consider to submit a V2 patch for this?

Yes, I think so.

> 
>> I'm not sure that it's really safe to access "slab->slabs" here without any protection?
>> Although it should be no problem in practice, alternative choice maybe putting partial
>> slabs count in the kmem_cache_cpu struct.
> 
> I think it is ok, because it seems that slab->slabs in slub_percpu_partial
> and show_slab_objects() are just used for showing some infomation.
> 
> I noticed Paul summarized some of these strategies in access-marking.txt[1]

Ok, thanks.

> 
> Quote from it:
> 
> "Use of the data_race() Macro
> ----------------------------
> 
> Here are some situations where data_race() should be used instead of
> READ_ONCE() and WRITE_ONCE():
> 
> 1.	Data-racy loads from shared variables whose values are used only
> 	for diagnostic purposes.
> 
> 2.	Data-racy reads whose values are checked against marked reload.
> 
> 3.	Reads whose values feed into error-tolerant heuristics.
> 
> 4.	Writes setting values that feed into error-tolerant heuristics.
> "
> 
> Thanks,
> Linke
> 
> [1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt
>