Re: [PATCH v2 02/12] block: prevent race condition on bi_status in __bio_chain_endio

Caleb Sander Mateos <[email protected]> 于2025年11月29日周六 03:44写道：
>
> On Fri, Nov 28, 2025 at 12:34 AM zhangshida <[email protected]> wrote:
> >
> > From: Shida Zhang <[email protected]>
> >
> > 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 <[email protected]>
> > Suggested-by: Christoph Hellwig <[email protected]>
> > Signed-off-by: Shida Zhang <[email protected]>
> > ---
> >  block/bio.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/block/bio.c b/block/bio.c
> > index 55c2c1a0020..aa43435c15f 100644
> > --- a/block/bio.c
> > +++ b/block/bio.c
> > @@ -313,9 +313,12 @@ EXPORT_SYMBOL(bio_reset);
> >  static struct bio *__bio_chain_endio(struct bio *bio)
> >  {
> >         struct bio *parent = bio->bi_private;
> > +       blk_status_t *status = &parent->bi_status;
>
> nit: this variable seems unnecessary, just use &parent->bi_status
> directly in the one place it's needed?
>

Thanks, Caleb and Andreas. I will integrate your suggestions to:

      if (!bio->bi_status)
               cmpxchg(&parent->bi_status, 0, bio->bi_status);

Thanks,
Shida

> Best,
> Caleb
>
> > +       blk_status_t new_status = bio->bi_status;
> > +
> > +       if (new_status != BLK_STS_OK)
> > +               cmpxchg(status, BLK_STS_OK, new_status);
> >
> > -       if (bio->bi_status && !parent->bi_status)
> > -               parent->bi_status = bio->bi_status;
> >         bio_put(bio);
> >         return parent;
> >  }
> > --
> > 2.34.1
> >
> >