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Message-ID: <b272ce4a-a20c-43de-8080-db8ad14abaff@suse.cz>
Date: Fri, 11 Jul 2025 17:56:34 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Jeongjun Park <aha310510@...il.com>, dennis@...nel.org, tj@...nel.org,
cl@...two.org
Cc: akpm@...ux-foundation.org, roman.gushchin@...ux.dev, rientjes@...gle.com,
shakeel.butt@...ux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
syzbot+e5bd32b79413e86f389e@...kaller.appspotmail.com
Subject: Re: [PATCH v2] mm/percpu: prevent concurrency problem for
pcpu_nr_populated read with spin lock
On 7/3/25 08:56, Jeongjun Park wrote:
> pcpu_nr_pages() reads pcpu_nr_populated without any protection, which
> causes a data race between read/write.
>
> However, since this is an intended race, we should add a data_race
> annotation instead of add a spin lock.
>
> Reported-by: syzbot+e5bd32b79413e86f389e@...kaller.appspotmail.com
> Fixes: 7e8a6304d541 ("/proc/meminfo: add percpu populated pages count")
> Suggested-by: Shakeel Butt <shakeel.butt@...ux.dev>
> Signed-off-by: Jeongjun Park <aha310510@...il.com>
> ---
> v2: Change it as suggested by Shakeel Butt
> - Link to v1: https://lore.kernel.org/all/20250702082749.141616-1-aha310510@gmail.com/
> ---
> mm/percpu.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/percpu.c b/mm/percpu.c
> index b35494c8ede2..782cc148b39c 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -3355,7 +3355,7 @@ void __init setup_per_cpu_areas(void)
> */
> unsigned long pcpu_nr_pages(void)
> {
> - return pcpu_nr_populated * pcpu_nr_units;
> + return data_race(READ_ONCE(pcpu_nr_populated) * pcpu_nr_units);
Nit: pcpu_nr_units should be excluded from the scope of data_race() as no
race can happen there.
> }
>
> /*
> --
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