linux-kernel - Re: [PATCH v4 3/3] block: prevent race condition on bi_status in __bio_chain

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Message-ID: <CAHc6FU4o8Wv+6TQti4NZJRUQpGF9RWqiN9fO6j55p4xgysM_3g@mail.gmail.com>
Date: Mon, 1 Dec 2025 11:22:32 +0100
From: Andreas Gruenbacher <agruenba@...hat.com>
To: zhangshida <starzhangzsd@...il.com>
Cc: Johannes.Thumshirn@....com, hch@...radead.org, ming.lei@...hat.com, 
	hsiangkao@...ux.alibaba.com, csander@...estorage.com, colyli@...as.com, 
	linux-block@...r.kernel.org, linux-bcache@...r.kernel.org, 
	linux-kernel@...r.kernel.org, zhangshida@...inos.cn, 
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v4 3/3] block: prevent race condition on bi_status in __bio_chain_endio

On Mon, Dec 1, 2025 at 10:05 AM zhangshida <starzhangzsd@...il.com> wrote:
> From: Shida Zhang <zhangshida@...inos.cn>
>
> Andreas point out that multiple completions can race setting
> bi_status.
>
> The check (parent->bi_status) and the subsequent write are not an
> atomic operation. The value of parent->bi_status could have changed
> between the time you read it for the if check and the time you write
> to it. So we use cmpxchg to fix the race, as suggested by Christoph.
>
> Suggested-by: Andreas Gruenbacher <agruenba@...hat.com>
> Suggested-by: Christoph Hellwig <hch@...radead.org>
> Suggested-by: Caleb Sander Mateos <csander@...estorage.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Shida Zhang <zhangshida@...inos.cn>
> ---
>  block/bio.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 1b5e4577f4c..097c1cd2054 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -314,8 +314,9 @@ static struct bio *__bio_chain_endio(struct bio *bio)
>  {
>         struct bio *parent = bio->bi_private;
>
> -       if (bio->bi_status && !parent->bi_status)
> -               parent->bi_status = bio->bi_status;
> +       if (bio->bi_status)
> +               cmpxchg(&parent->bi_status, 0, bio->bi_status);

Hmm. I don't think cmpxchg() actually is of any value here: for all
the chained bios, bi_status is initialized to 0, and it is only set
again (to a non-0 value) when a failure occurs. When there are
multiple failures, we only need to make sure that one of those
failures is eventually reported, but for that, a simple assignment is
enough here. The cmpxchg() won't guarantee that a specific error value
will survive; it all still depends on the timing. The cmpxchg() only
makes it look like something special is happening here with respect to
ordering.

> +
>         bio_put(bio);
>         return parent;
>  }
> --
> 2.34.1
>

Thanks,
Andreas