From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Snitzer Subject: Re: block: be more careful about status in __bio_chain_endio Date: Fri, 22 Feb 2019 16:10:06 -0500 Message-ID: <20190222211006.GA10987@redhat.com> References: <70cda2a3-f246-d45b-f600-1f9d15ba22ff@gmail.com> <87eflmpqkb.fsf@notabene.neil.brown.name> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <87eflmpqkb.fsf@notabene.neil.brown.name> Sender: linux-kernel-owner@vger.kernel.org To: NeilBrown Cc: Jens Axboe , linux-block@vger.kernel.org, device-mapper development , Milan Broz , Linux Kernel Mailing List List-Id: dm-devel.ids On Thu, Feb 15 2018 at 4:09am -0500, NeilBrown 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 > --- > 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 Jens, this one slipped through the crack just over a year ago. It is available in patchwork here: https://patchwork.kernel.org/patch/10220727/