linux-kernel - Re: [PATCH -next] mm/percpu: fix data-race with pcpu_nr_empty_pop

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Date:   Mon, 25 Oct 2021 09:50:48 +0200 (CEST)
From:   Christoph Lameter <cl@...two.de>
To:     Yuanzheng Song <songyuanzheng@...wei.com>
cc:     dennis@...nel.org, tj@...nel.org, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next] mm/percpu: fix data-race with
 pcpu_nr_empty_pop_pages

On Mon, 25 Oct 2021, Yuanzheng Song wrote:

> When reading the pcpu_nr_empty_pop_pages in pcpu_alloc()
> and writing the pcpu_nr_empty_pop_pages in
> pcpu_update_empty_pages() at the same time,
> the data-race occurs.

Looks like a use case for the atomic RMV instructions.

> To fix this issue, use READ_ONCE() and WRITE_ONCE() to
> read and write the pcpu_nr_empty_pop_pages.

Never thought that READ_ONCE and WRITE_ONCE can fix races like
this. Really?

> diff --git a/mm/percpu.c b/mm/percpu.c
> index 293009cc03ef..e8ef92e698ab 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -574,7 +574,9 @@ static void pcpu_isolate_chunk(struct pcpu_chunk *chunk)
>
>  	if (!chunk->isolated) {
>  		chunk->isolated = true;
> -		pcpu_nr_empty_pop_pages -= chunk->nr_empty_pop_pages;
> +		WRITE_ONCE(pcpu_nr_empty_pop_pages,
> +			   READ_ONCE(pcpu_nr_empty_pop_pages) -
> +			   chunk->nr_empty_pop_pages);

atomic_sub()?

>  	}
>  	list_move(&chunk->list, &pcpu_chunk_lists[pcpu_to_depopulate_slot]);
>  }
> @@ -585,7 +587,9 @@ static void pcpu_reintegrate_chunk(struct pcpu_chunk *chunk)
>
>  	if (chunk->isolated) {
>  		chunk->isolated = false;
> -		pcpu_nr_empty_pop_pages += chunk->nr_empty_pop_pages;
> +		WRITE_ONCE(pcpu_nr_empty_pop_pages,
> +			   READ_ONCE(pcpu_nr_empty_pop_pages) +
> +			   chunk->nr_empty_pop_pages);

atomic_add()?