Re: [PATCH] mm/percpu: prevent concurrency problem for pcpu_nr_populated read with spin lock

[Shakeel Butt <shakeel.butt@linux.dev>]

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@syzkaller.appspotmail.com
> Fixes: 7e8a6304d541 ("/proc/meminfo: add percpu populated pages count")
> Signed-off-by: Jeongjun Park <aha310510@gmail.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;