[PATCH] blkcg: stop iteration early if root_rl is the only request list

["Jun'ichi Nomura" <j-nomura@ce.jp.nec.com>]

__blk_queue_next_rl() finds next request list based on blkg_list
while skipping root_blkg in the list.
OTOH, root_rl is special as it may exist even without root_blkg.

Though the later part of the function handles such a case correctly,
exiting early is good for readability of the code.

Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
---
 block/blk-cgroup.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 54f35d1..a31e678 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -333,6 +333,9 @@ struct request_list *__blk_queue_next_rl(struct request_list *rl,
 	 */
 	if (rl == &q->root_rl) {
 		ent = &q->blkg_list;
+		/* There are no more block groups, hence no request lists */
+		if (list_empty(ent))
+			return NULL;
 	} else {
 		blkg = container_of(rl, struct blkcg_gq, rl);
 		ent = &blkg->q_node;