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

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Message-ID: <7b7d353f-f38b-3205-8fd4-1072dbf69cb6@gentwo.org>
Date: Wed, 2 Jul 2025 08:55:13 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Jeongjun Park <aha310510@...il.com>
cc: dennis@...nel.org, tj@...nel.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, 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?