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

[Christoph Lameter (Ampere)]

On Wed, 2 Jul 2025, Jeongjun Park wrote:

> 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;
> +	unsigned long flags, ret;
> +
> +	spin_lock_irqsave(&pcpu_lock, flags);
> +	ret = pcpu_nr_populated * pcpu_nr_units;
> +	spin_unlock_irqrestore(&pcpu_lock, flags);


Ummm.. What? You are protecting a single read with a spinlock? There needs
to be some updating of data somewhere for this to make sense.


Unless a different critical section protected by the lock sets the value
intermittendly to something you are not allowed to see before a final
store of a valid value. But that would be unusual.

This is an academic exercise or did you really see a problem?

What is racing?