[PATCH] bcache: fix double bio_endio completion in detached_dev_end_io

[zhangshida]

From: Shida Zhang <zhangshida@xxxxxxxxxx>

Commit 53280e398471 ("bcache: fix improper use of bi_end_io") attempted
to fix up bio completions by replacing manual bi_end_io calls with
bio_endio(). However, it introduced a double-completion bug in the
detached_dev path.

In a normal completion path, the call stack is:
   blk_update_request
     bio_endio(bio)
       bio->bi_end_io(bio) -> detached_dev_end_io
         bio_endio(bio)    <- BUG: second call

To fix this, detached_dev_end_io() must manually call the next completion
handler in the chain.

However, in detached_dev_do_request(), if a discard is unsupported, the
bio is rejected before being submitted to the lower level. In this case,
we can use the standard bio_endio().

   detached_dev_do_request
     bio_endio(bio)        <- Correct: starts completion for
				unsubmitted bio

Fixes: 53280e398471 ("bcache: fix improper use of bi_end_io")
Signed-off-by: Shida Zhang <zhangshida@xxxxxxxxxx>
---
 drivers/md/bcache/request.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/md/bcache/request.c b/drivers/md/bcache/request.c
index 82fdea7dea7..ec712b5879f 100644
--- a/drivers/md/bcache/request.c
+++ b/drivers/md/bcache/request.c
@@ -1104,7 +1104,14 @@ static void detached_dev_end_io(struct bio *bio)
 	}
 
 	kfree(ddip);
-	bio_endio(bio);
+	/*
+	 * This is an exception where bio_endio() cannot be used.
+	 * We are already called from within a bio_endio() stack;
+	 * calling it again here would result in a double-completion
+	 * (decrementing bi_remaining twice). We must call the
+	 * original completion routine directly.
+	 */
+	bio->bi_end_io(bio);
 }
 
 static void detached_dev_do_request(struct bcache_device *d, struct bio *bio,
@@ -1136,7 +1143,7 @@ static void detached_dev_do_request(struct bcache_device *d, struct bio *bio,
 
 	if ((bio_op(bio) == REQ_OP_DISCARD) &&
 	    !bdev_max_discard_sectors(dc->bdev))
-		detached_dev_end_io(bio);
+		bio_endio(bio);
 	else
 		submit_bio_noacct(bio);
 }
-- 
2.34.1