dm: correctly handle chained bios in dec_pending()

Web:        https://git.kernel.org/torvalds/c/8dd601fa8317243be887458c49f6c29c2f3d719f
Commit:     8dd601fa8317243be887458c49f6c29c2f3d719f
Parent:     7928b2cbe55b2a410a0f5c1f154610059c57b1b2
Refname:    refs/heads/master
Author:     NeilBrown <[email protected]>
AuthorDate: Thu Feb 15 20:00:15 2018 +1100
Committer:  Mike Snitzer <[email protected]>
CommitDate: Fri Feb 16 10:46:35 2018 -0500

    dm: correctly handle chained bios in dec_pending()
    
    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 0.
    
    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
    has 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 <[email protected]>
    Cc: [email protected] (v3.14+)
    Signed-off-by: NeilBrown <[email protected]>
    Signed-off-by: Mike Snitzer <[email protected]>
---
 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);
 		}
 	}
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