Re: block: be more careful about status in __bio_chain_endio

On Fri, Feb 22 2019 at  5:46pm -0500,
Jens Axboe [off-list ref] wrote:

On 2/22/19 2:10 PM, Mike Snitzer wrote:

quoted

On Thu, Feb 15 2018 at  4:09am -0500,
NeilBrown [off-list ref] wrote:

quoted

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 <redacted>
---
 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 <redacted>

Jens, this one slipped through the crack just over a year ago.
It is available in patchwork here:
https://patchwork.kernel.org/patch/10220727/

Should this be:

	if (!parent->bi_status && bio->bi_status)
		parent->bi_status = bio->bi_status;

perhaps?

Yeap, even better.  Not seeing any reason to have the last error win,
the first in the chain is likely the most important.