On Thu, Feb 15 2018 at 4:09am -0500,
NeilBrown <neilb@xxxxxxxx> 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@xxxxxxxx>
> ---
> 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@xxxxxxxxxx>
Jens, this one slipped through the crack just over a year ago.
It is available in patchwork here:
https://patchwork.kernel.org/patch/10220727/
