Re: [PATCH v2] mm/percpu: prevent concurrency problem for pcpu_nr_populated read with spin lock

[Vlastimil Babka <vbabka@suse.cz>]

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@syzkaller.appspotmail.com
> Fixes: 7e8a6304d541 ("/proc/meminfo: add percpu populated pages count")
> Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
> Signed-off-by: Jeongjun Park <aha310510@gmail.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.

>  }
>  
>  /*
> --