From: Mike Snitzer <snitzer@redhat.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: NeilBrown <neilb@suse.com>,
linux-block@vger.kernel.org,
device-mapper development <dm-devel@redhat.com>,
Milan Broz <gmazyland@gmail.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: block: be more careful about status in __bio_chain_endio
Date: Fri, 22 Feb 2019 18:55:00 -0500 [thread overview]
Message-ID: <20190222235459.GA11726@redhat.com> (raw)
In-Reply-To: <7f0aeb7b-fdaa-0625-f785-05c342047550@kernel.dk>
On Fri, Feb 22 2019 at 5:46pm -0500,
Jens Axboe <axboe@kernel.dk> wrote:
> On 2/22/19 2:10 PM, Mike Snitzer wrote:
> > On Thu, Feb 15 2018 at 4:09am -0500,
> > NeilBrown <neilb@suse.com> wrote:
> >
> >>
> >> If two bios are chained under the one parent (with bio_chain())
> >> it is possible that one will succeed and the other will fail.
> >> __bio_chain_endio must ensure that the failure error status
> >> is reported for the whole, rather than the success.
> >>
> >> It currently tries to be careful, but this test is racy.
> >> If both children finish at the same time, they might both see that
> >> parent->bi_status as zero, and so will assign their own status.
> >> If the assignment to parent->bi_status by the successful bio happens
> >> last, the error status will be lost which can lead to silent data
> >> corruption.
> >>
> >> Instead, __bio_chain_endio should only assign a non-zero status
> >> to parent->bi_status. There is then no need to test the current
> >> value of parent->bi_status - a test that would be racy anyway.
> >>
> >> Note that this bug hasn't been seen in practice. It was only discovered
> >> by examination after a similar bug was found in dm.c
> >>
> >> Signed-off-by: NeilBrown <neilb@suse.com>
> >> ---
> >> block/bio.c | 2 +-
> >> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/block/bio.c b/block/bio.c
> >> index e1708db48258..ad77140edc6f 100644
> >> --- a/block/bio.c
> >> +++ b/block/bio.c
> >> @@ -312,7 +312,7 @@ static struct bio *__bio_chain_endio(struct bio *bio)
> >> {
> >> struct bio *parent = bio->bi_private;
> >>
> >> - if (!parent->bi_status)
> >> + if (bio->bi_status)
> >> parent->bi_status = bio->bi_status;
> >> bio_put(bio);
> >> return parent;
> >> --
> >> 2.14.0.rc0.dirty
> >>
> >
> > Reviewed-by: Mike Snitzer <snitzer@redhat.com>
> >
> > Jens, this one slipped through the crack just over a year ago.
> > It is available in patchwork here:
> > https://patchwork.kernel.org/patch/10220727/
>
> Should this be:
>
> if (!parent->bi_status && bio->bi_status)
> parent->bi_status = bio->bi_status;
>
> perhaps?
Yeap, even better. Not seeing any reason to have the last error win,
the first in the chain is likely the most important.
next prev parent reply other threads:[~2019-02-22 23:55 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <70cda2a3-f246-d45b-f600-1f9d15ba22ff@gmail.com>
[not found] ` <87eflmpqkb.fsf@notabene.neil.brown.name>
2019-02-22 21:10 ` block: be more careful about status in __bio_chain_endio Mike Snitzer
2019-02-22 22:46 ` Jens Axboe
2019-02-22 23:55 ` Mike Snitzer [this message]
2019-02-23 2:02 ` John Dorminy
2019-02-23 2:44 ` Mike Snitzer
2019-02-23 3:10 ` John Dorminy
2019-06-12 2:56 ` John Dorminy
2019-06-12 7:01 ` Christoph Hellwig
2019-06-17 7:32 ` Hannes Reinecke
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