Re: [PATCH v6 3/3] block: prevent race condition on bi_status in __bio_chain_endio

[Stephen Zhang]

Andreas Gruenbacher <agruenba@xxxxxxxxxx> 于2025年12月7日周日 21:30写道：
>
> On Sun, Dec 7, 2025 at 1:22 PM zhangshida <starzhangzsd@xxxxxxxxx> wrote:
> > From: Shida Zhang <zhangshida@xxxxxxxxxx>
> >
> > Andreas point out that multiple completions can race setting
> > bi_status.
> >
> > If __bio_chain_endio() is called concurrently from multiple threads
> > accessing the same parent bio, it should use WRITE_ONCE()/READ_ONCE()
> > to access parent->bi_status and avoid data races.
> >
> > On x86 and ARM, these macros compile to the same instruction as a
> > normal write, but they may be required on other architectures to
> > prevent tearing, and to ensure the compiler does not add or remove
> > memory accesses under the assumption that the values are not accessed
> > concurrently.
> >
> > Adopting a cmpxchg approach, as used in other code paths, resolves all
> > these issues, as suggested by Christoph.
> >
> > Suggested-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
> > Suggested-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
> > Suggested-by: Caleb Sander Mateos <csander@xxxxxxxxxxxxxxx>
> > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > Signed-off-by: Shida Zhang <zhangshida@xxxxxxxxxx>
> > ---
> >  block/bio.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/block/bio.c b/block/bio.c
> > index d236ca35271..8b4b6b4e210 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);
> > +
> >         bio_put(bio);
> >         return parent;
> >  }
> > --
> > 2.34.1
> >
>
> I thought you were going to drop this??
>

Okay, I will drop it.

Thanks,
Shida

> Andreas
>