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

Li Zefan · Jens Axboe · commit 0341509fdfc9 · 2010-05-07T08:57:00.000+02:00
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 &lt;lizf@cn.fujitsu.com&gt;
Acked-by: Vivek Goyal &lt;vgoyal@redhat.com&gt;
Signed-off-by: Jens Axboe &lt;jens.axboe@oracle.com&gt;
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
@@ -286,16 +286,16 @@ static void blkiocg_destroy(struct cgroup_subsys *subsys, struct cgroup *cgroup)
 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);
