linux-kernel - Re: [PATCH -next v2 01/28] md: use READ_ONCE/WRITE_ONCE for 'suspend_lo' and 'suspend

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Message-ID: <CALTww28MiiWTOyLYHErAZWTzn8iGif5=adY7yohxmn1OxrpK=w@mail.gmail.com>
Date:   Thu, 14 Sep 2023 10:53:41 +0800
From:   Xiao Ni <xni@...hat.com>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     agk@...hat.com, snitzer@...nel.org, dm-devel@...hat.com,
        song@...nel.org, linux-kernel@...r.kernel.org,
        linux-raid@...r.kernel.org, yukuai3@...wei.com,
        yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH -next v2 01/28] md: use READ_ONCE/WRITE_ONCE for
 'suspend_lo' and 'suspend_hi'

On Mon, Aug 28, 2023 at 10:04 AM Yu Kuai <yukuai1@...weicloud.com> wrote:
>
> From: Yu Kuai <yukuai3@...wei.com>
>
> Because reading 'suspend_lo' and 'suspend_hi' from md_handle_request()
> is not protected, use READ_ONCE/WRITE_ONCE to prevent reading abnormal
> value.

Hi Kuai

If we don't use READ_ONCE/WRITE_ONCE, What's the risk here? Could you
explain in detail or give an example?

Regards
Xiao
>
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  drivers/md/md.c | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 46badd13a687..9d8dff9d923c 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -359,11 +359,11 @@ static bool is_suspended(struct mddev *mddev, struct bio *bio)
>                 return true;
>         if (bio_data_dir(bio) != WRITE)
>                 return false;
> -       if (mddev->suspend_lo >= mddev->suspend_hi)
> +       if (READ_ONCE(mddev->suspend_lo) >= READ_ONCE(mddev->suspend_hi))
>                 return false;
> -       if (bio->bi_iter.bi_sector >= mddev->suspend_hi)
> +       if (bio->bi_iter.bi_sector >= READ_ONCE(mddev->suspend_hi))
>                 return false;
> -       if (bio_end_sector(bio) < mddev->suspend_lo)
> +       if (bio_end_sector(bio) < READ_ONCE(mddev->suspend_lo))
>                 return false;
>         return true;
>  }
> @@ -5171,7 +5171,8 @@ __ATTR(sync_max, S_IRUGO|S_IWUSR, max_sync_show, max_sync_store);
>  static ssize_t
>  suspend_lo_show(struct mddev *mddev, char *page)
>  {
> -       return sprintf(page, "%llu\n", (unsigned long long)mddev->suspend_lo);
> +       return sprintf(page, "%llu\n",
> +                      (unsigned long long)READ_ONCE(mddev->suspend_lo));
>  }
>
>  static ssize_t
> @@ -5191,7 +5192,7 @@ suspend_lo_store(struct mddev *mddev, const char *buf, size_t len)
>                 return err;
>
>         mddev_suspend(mddev);
> -       mddev->suspend_lo = new;
> +       WRITE_ONCE(mddev->suspend_lo, new);
>         mddev_resume(mddev);
>
>         mddev_unlock(mddev);
> @@ -5203,7 +5204,8 @@ __ATTR(suspend_lo, S_IRUGO|S_IWUSR, suspend_lo_show, suspend_lo_store);
>  static ssize_t
>  suspend_hi_show(struct mddev *mddev, char *page)
>  {
> -       return sprintf(page, "%llu\n", (unsigned long long)mddev->suspend_hi);
> +       return sprintf(page, "%llu\n",
> +                      (unsigned long long)READ_ONCE(mddev->suspend_hi));
>  }
>
>  static ssize_t
> @@ -5223,7 +5225,7 @@ suspend_hi_store(struct mddev *mddev, const char *buf, size_t len)
>                 return err;
>
>         mddev_suspend(mddev);
> -       mddev->suspend_hi = new;
> +       WRITE_ONCE(mddev->suspend_hi, new);
>         mddev_resume(mddev);
>
>         mddev_unlock(mddev);
> --
> 2.39.2
>