blk-cgroup: Fix an RCU warning in blkiocg_create()

with CONFIG_PROVE_RCU=y, a warning can be triggered:

  # mount -t cgroup -o blkio xxx /mnt
  # mkdir /mnt/subgroup

...
kernel/cgroup.c:4442 invoked rcu_dereference_check() without protection!
...

To fix this, we avoid caling css_depth() here, which is a bit simpler
than the original code.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 5fe03def34b2..2cc682b860ea 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -286,16 +286,16 @@ done:
 static struct cgroup_subsys_state *
 blkiocg_create(struct cgroup_subsys *subsys, struct cgroup *cgroup)
 {
-        struct blkio_cgroup *blkcg, *parent_blkcg;
+        struct blkio_cgroup *blkcg;
+        struct cgroup *parent = cgroup->parent;
 
-        if (!cgroup->parent) {
+        if (!parent) {
                 blkcg = &blkio_root_cgroup;
                 goto done;
         }
 
         /* Currently we do not support hierarchy deeper than two level (0,1) */
-        parent_blkcg = cgroup_to_blkio_cgroup(cgroup->parent);
-        if (css_depth(&parent_blkcg->css) > 0)
+        if (parent != cgroup->top_cgroup)
                 return ERR_PTR(-EINVAL);
 
         blkcg = kzalloc(sizeof(*blkcg), GFP_KERNEL);