linux-kernel - Re: [PATCH v2 02/12] block: prevent race condition on bi_status in __bio_chain

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Message-ID: <CANubcdU2f1+fCL9sYsrwXz-W0dzEsm_+Bds1m3W8=o_sQX30Hw@mail.gmail.com>
Date: Sat, 29 Nov 2025 10:32:38 +0800
From: Stephen Zhang <starzhangzsd@...il.com>
To: Caleb Sander Mateos <csander@...estorage.com>
Cc: Johannes.Thumshirn@....com, hch@...radead.org, gruenba@...hat.com, 
	ming.lei@...hat.com, linux-block@...r.kernel.org, 
	linux-bcache@...r.kernel.org, nvdimm@...ts.linux.dev, 
	virtualization@...ts.linux.dev, linux-nvme@...ts.infradead.org, 
	gfs2@...ts.linux.dev, ntfs3@...ts.linux.dev, linux-xfs@...r.kernel.org, 
	linux-kernel@...r.kernel.org, zhangshida@...inos.cn, 
	Andreas Gruenbacher <agruenba@...hat.com>, Gao Xiang <hsiangkao@...ux.alibaba.com>
Subject: Re: [PATCH v2 02/12] block: prevent race condition on bi_status in __bio_chain_endio

Stephen Zhang <starzhangzsd@...il.com> 于2025年11月29日周六 09:47写道：
>
> Caleb Sander Mateos <csander@...estorage.com> 于2025年11月29日周六 03:44写道：
> >
> > On Fri, Nov 28, 2025 at 12:34 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>
> > > Signed-off-by: Shida Zhang <zhangshida@...inos.cn>
> > > ---
> > >  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);
>

Sorry, it should be:
      if (bio->bi_status)
              cmpxchg(&parent->bi_status, 0, bio->bi_status);

Thanks,
Shida

> 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
> > >
> > >