linux-kernel - [PATCH 5/5] blk-cgroup: Fix an RCU warning in blkiocg

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Date:	Thu, 22 Apr 2010 17:32:28 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	LKML <linux-kernel@...r.kernel.org>
CC:	"containers@...ts.osdl.org" <containers@...ts.osdl.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Gui Jianfeng <GuiJianfeng@...fujitsu.com>
Subject: [PATCH 5/5] 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@...fujitsu.com>
---
 block/blk-cgroup.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 5fe03de..2cc682b 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);
-- 
1.6.3

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