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>

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 5fe03de..2cc682b 100644 (file)
--- 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);