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 <gmazyl...@gmail.com>
Cc: sta...@vger.kernel.org (v3.14+)
Signed-off-by: NeilBrown <ne...@suse.com>
---
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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