[PATCH 5/8] blk-cgroup: don't nest queue_lock under rcu in bio_associate_blkg()

[Yu Kuai]

From: Yu Kuai <yukuai@xxxxxxx>

If a bio is already associated with a blkg, the blkcg is already pinned
until the bio is done, so there is no need for RCU protection. Otherwise,
protect blkcg_css() with RCU independently. Prepare to protect blkcg with
blkcg_mutex instead of queue_lock.

Signed-off-by: Yu Kuai <yukuai@xxxxxxx>
---
 block/blk-cgroup.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index e2896d582235..8c9ca52a54f4 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -2186,20 +2186,24 @@ void bio_associate_blkg(struct bio *bio)
 	struct cgroup_subsys_state *css;
 
 	if (blk_op_is_passthrough(bio->bi_opf))
 		return;
 
-	rcu_read_lock();
-
-	if (bio->bi_blkg)
+	if (bio->bi_blkg) {
 		css = bio_blkcg_css(bio);
-	else
+		bio_associate_blkg_from_css(bio, css);
+	} else {
+		rcu_read_lock();
 		css = blkcg_css();
+		if (!css_tryget_online(css))
+			css = NULL;
+		rcu_read_unlock();
 
-	bio_associate_blkg_from_css(bio, css);
-
-	rcu_read_unlock();
+		bio_associate_blkg_from_css(bio, css);
+		if (css)
+			css_put(css);
+	}
 }
 EXPORT_SYMBOL_GPL(bio_associate_blkg);
 
 /**
  * bio_clone_blkg_association - clone blkg association from src to dst bio
-- 
2.51.0