[PATCH] dm: correctly handle chained bios in dec_pending()

[NeilBrown]

dec_pending() is given an error status (possibly 0)
to be recorded against a bio.
It can be called several times on the one 'struct dm_io',
and it is careful to only assign a non-zero error to
io->status.
However when it then assigned io->status to bio->bi_status,
it is not careful and could overwrite a genuine error status with
zero.

This can happen when chained bios are in use.  If a bio is chained
beneath the bio that this dm_io is handling, the child bio might
complete and set bio->bi_status before the dm_io completes.

This has been possible since chained bios were introduced in 3.14, and
become a lot easier to trigger with commit 18a25da84354 ("dm: ensure bio
submission follows a depth-first tree walk") as that commit caused dm to
start using chained bios itself.

A particular failure mode is that if a bio spans an 'error' target and
a working target, the 'error' fragment will complete instantly and set
the ->bi_status, and the other fragment will normally complete a
little later, and will clear ->bi_status.

The fix is simply to only assign io_error to bio->bi_status when
io_error is not zero.

Reported-and-tested-by: Milan Broz <gmazyland@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx (v3.14+)
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
 drivers/md/dm.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index d6de00f367ef..68136806d365 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -903,7 +903,8 @@ static void dec_pending(struct dm_io *io, blk_status_t error)
 			queue_io(md, bio);
 		} else {
 			/* done with normal IO or empty flush */
-			bio->bi_status = io_error;
+			if (io_error)
+				bio->bi_status = io_error;
 			bio_endio(bio);
 		}
 	}
-- 
2.14.0.rc0.dirty

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