block: bypass blkg_tryget_closest for the root_blkg

The root_blkg is only torn down at the very end of removing a queue.
So in the I/O submission path is always has a life reference and we
can just grab another one using blkg_get instead of doing a tryget
and parent walk that won't lead anywhere.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 6aedb73..0912820 100644 (file)
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1758,16 +1758,21 @@ void bio_associate_blkg_from_css(struct bio *bio,
                                 struct cgroup_subsys_state *css)
 {
        struct request_queue *q = bio->bi_disk->queue;
-       struct blkcg_gq *blkg = q->root_blkg;
 
        if (bio->bi_blkg)
                blkg_put(bio->bi_blkg);
 
-       rcu_read_lock();
-       if (css && css->parent)
+       if (css && css->parent) {
+               struct blkcg_gq *blkg;
+
+               rcu_read_lock();
                blkg = blkg_lookup_create(css_to_blkcg(css), q);
-       bio->bi_blkg = blkg_tryget_closest(blkg);
-       rcu_read_unlock();
+               bio->bi_blkg = blkg_tryget_closest(blkg);
+               rcu_read_unlock();
+       } else {
+               blkg_get(q->root_blkg);
+               bio->bi_blkg = q->root_blkg;
+       }
 }
 EXPORT_SYMBOL_GPL(bio_associate_blkg_from_css);