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

[Yu Kuai <yukuai1@xxxxxxxxxxxxxxx>]

Hi,

在 2023/09/14 10:53, Xiao Ni 写道:
> On Mon, Aug 28, 2023 at 10:04 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > 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?

Sorry for the late reply.

That depends on the architecture, a load/store may not be atomice,
for example:

// assume a is 10
t1 write 01
// write half first
a = 11
		t2 read
		//read
		a = 11 -> read abnormal value.
// write other half
a = 01

READ_ONCE/WRITE_ONCE can guarantee that either old value or new value is
read.

Thanks,
Kuai

> Regards
> Xiao
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > ---
> >   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
>
> .