[PATCH v2 02/12] block: prevent race condition on bi_status in __bio_chain_endio

[zhangshida]

From: Shida Zhang <zhangshida at kylinos.cn>

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 <agruenba at redhat.com>
Suggested-by: Christoph Hellwig <hch at infradead.org>
Signed-off-by: Shida Zhang <zhangshida at kylinos.cn>
---
 block/bio.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 55c2c1a0020..aa43435c15f 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -313,9 +313,12 @@ EXPORT_SYMBOL(bio_reset);
 static struct bio *__bio_chain_endio(struct bio *bio)
 {
 	struct bio *parent = bio->bi_private;
+	blk_status_t *status = &parent->bi_status;
+	blk_status_t new_status = bio->bi_status;
+
+	if (new_status != BLK_STS_OK)
+		cmpxchg(status, BLK_STS_OK, new_status);
 
-	if (bio->bi_status && !parent->bi_status)
-		parent->bi_status = bio->bi_status;
 	bio_put(bio);
 	return parent;
 }
-- 
2.34.1