[PATCH 1/3] cgroup: make sure parent won't be destroyed before its children

Tejun Heo tj at kernel.org
Tue Apr 9 02:23:31 UTC 2013

From: Li Zefan <lizefan at huawei.com>

Suppose we rmdir a cgroup and there're still css refs, this cgroup won't
be freed. Then we rmdir the parent cgroup, and the parent is freed
immediately due to css ref draining to 0. Now it would be a disaster if
the still-alive child cgroup tries to access its parent.

Make sure this won't happen.

Signed-off-by: Li Zefan <lizefan at huawei.com>
Reviewed-by: Michal Hocko <mhocko at suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
Signed-off-by: Tejun Heo <tj at kernel.org>
 kernel/cgroup.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index ba3e24a..d7dfa64 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -876,6 +876,13 @@ static void cgroup_free_fn(struct work_struct *work)
+	 * We get a ref to the parent's dentry, and put the ref when
+	 * this cgroup is being freed, so it's guaranteed that the
+	 * parent won't be destroyed before its children.
+	 */
+	dput(cgrp->parent->dentry);
+	/*
 	 * Drop the active superblock reference that we took when we
 	 * created the cgroup
@@ -4168,6 +4175,9 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 	for_each_subsys(root, ss)
+	/* hold a ref to the parent's dentry */
+	dget(parent->dentry);
 	/* creation succeeded, notify subsystems */
 	for_each_subsys(root, ss) {
 		err = online_css(ss, cgrp);

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