block: free merged request in the caller

If we end up doing a request-to-request merge when we have completed
a bio-to-request merge, we free the request from deep down in that
path. For blk-mq-sched, the merge path has to hold the appropriate
lock, but we don't need it for freeing the request. And in fact
holding the lock is problematic, since we are now calling the
mq sched put_rq_private() hook with the lock held. Other call paths
do not hold this lock.

Fix this inconsistency by ensuring that the caller frees a merged
request. Then we can do it outside of the lock, making it both more
efficient and fixing the blk-mq-sched problem of invoking parts of
the scheduler with an unknown lock state.

Reported-by: Paolo Valente <paolo.valente@linaro.org>
Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index 76c42f559df8e..1d263311353ab 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1596,7 +1596,7 @@ static blk_qc_t blk_queue_bio(struct request_queue *q, struct bio *bio)
 {
 	struct blk_plug *plug;
 	int el_ret, where = ELEVATOR_INSERT_SORT;
-	struct request *req;
+	struct request *req, *free;
 	unsigned int request_count = 0;
 	unsigned int wb_acct;
 
@@ -1637,15 +1637,21 @@ static blk_qc_t blk_queue_bio(struct request_queue *q, struct bio *bio)
 	if (el_ret == ELEVATOR_BACK_MERGE) {
 		if (bio_attempt_back_merge(q, req, bio)) {
 			elv_bio_merged(q, req, bio);
-			if (!attempt_back_merge(q, req))
+			free = attempt_back_merge(q, req);
+			if (!free)
 				elv_merged_request(q, req, el_ret);
+			else
+				__blk_put_request(q, free);
 			goto out_unlock;
 		}
 	} else if (el_ret == ELEVATOR_FRONT_MERGE) {
 		if (bio_attempt_front_merge(q, req, bio)) {
 			elv_bio_merged(q, req, bio);
-			if (!attempt_front_merge(q, req))
+			free = attempt_front_merge(q, req);
+			if (!free)
 				elv_merged_request(q, req, el_ret);
+			else
+				__blk_put_request(q, free);
 			goto out_unlock;
 		}
 	}