[PATCH v6 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.

If __bio_chain_endio() is called concurrently from multiple threads
accessing the same parent bio, it should use WRITE_ONCE()/READ_ONCE()
to access parent->bi_status and avoid data races.

On x86 and ARM, these macros compile to the same instruction as a
normal write, but they may be required on other architectures to
prevent tearing, and to ensure the compiler does not add or remove
memory accesses under the assumption that the values are not accessed
concurrently.

Adopting a cmpxchg approach, as used in other code paths, resolves all
these issues, 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 d236ca35271..8b4b6b4e210 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