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Message-ID: <8bc1aebf-8395-416f-8c23-53cbd25d0eef@linux.dev>
Date: Sat, 9 Mar 2024 19:24:11 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: linke li <lilinke99@...com>
Cc: Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
Andrew Morton <akpm@...ux-foundation.org>, Vlastimil Babka <vbabka@...e.cz>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/slub: mark racy accesses on slab->slabs
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@...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.
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