cgroup: make sure parent won't be destroyed before its children

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@huawei.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index fd38e1cfaccae18f887cd32a393b565ebec58aea..65b72d0367c54f3949ec3181e714ee6d77f87b32 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -875,6 +875,13 @@ static void cgroup_free_fn(struct work_struct *work)
        cgrp->root->number_of_cgroups--;
        mutex_unlock(&cgroup_mutex);
 
+       /*
+        * 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
@@ -4164,6 +4171,9 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
        for_each_subsys(root, ss)
                dget(dentry);
 
+       /* 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);