[PATCH v4 3/3] block: prevent race condition on bi_status in __bio_chain_endio

From: Shida Zhang <[email protected]>

Andreas point out that multiple completions can race setting
bi_status.

The check (parent->bi_status) and the subsequent write are not an
atomic operation. The value of parent->bi_status could have changed
between the time you read it for the if check and the time you write
to it. So we use cmpxchg to fix the race, as suggested by Christoph.

Suggested-by: Andreas Gruenbacher <[email protected]>
Suggested-by: Christoph Hellwig <[email protected]>
Suggested-by: Caleb Sander Mateos <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Shida Zhang <[email protected]>
---
 block/bio.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 1b5e4577f4c..097c1cd2054 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -314,8 +314,9 @@ static struct bio *__bio_chain_endio(struct bio *bio)
 {
 	struct bio *parent = bio->bi_private;
 
-	if (bio->bi_status && !parent->bi_status)
-		parent->bi_status = bio->bi_status;
+	if (bio->bi_status)
+		cmpxchg(&parent->bi_status, 0, bio->bi_status);
+
 	bio_put(bio);
 	return parent;
 }
-- 
2.34.1