linux-kernel - Re: [PATCH] mm/percpu: prevent concurrency problem for pcpu_nr

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Message-ID: <zvczeiqk535pvryfae4lta5ezvqks6knhugnzgyq6gu634gn4c@av5hhiabqmzf>
Date: Wed, 2 Jul 2025 10:03:50 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Jeongjun Park <aha310510@...il.com>
Cc: dennis@...nel.org, tj@...nel.org, cl@...two.org, 
	akpm@...ux-foundation.org, vbabka@...e.cz, rientjes@...gle.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, syzbot+e5bd32b79413e86f389e@...kaller.appspotmail.com
Subject: Re: [PATCH] mm/percpu: prevent concurrency problem for
 pcpu_nr_populated read with spin lock

On Wed, Jul 02, 2025 at 05:27:49PM +0900, Jeongjun Park wrote:
> Read/Write to pcpu_nr_populated should be performed while protected
> by pcpu_lock. However, pcpu_nr_pages() reads pcpu_nr_populated without any
> protection, which causes a data race between read/write.
> 
> Therefore, when reading pcpu_nr_populated in pcpu_nr_pages(), it should be
> modified to be protected by pcpu_lock.
> 
> Reported-by: syzbot+e5bd32b79413e86f389e@...kaller.appspotmail.com
> Fixes: 7e8a6304d541 ("/proc/meminfo: add percpu populated pages count")
> Signed-off-by: Jeongjun Park <aha310510@...il.com>
> ---
>  mm/percpu.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index b35494c8ede2..0f98b857fb36 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -3355,7 +3355,13 @@ void __init setup_per_cpu_areas(void)
>   */
>  unsigned long pcpu_nr_pages(void)
>  {
> -	return pcpu_nr_populated * pcpu_nr_units;

No need for the lock as I think race is fine here. Use something like
the following and add a comment.

	data_race(READ_ONCE(pcpu_nr_populated)) * pcpu_nr_units;