[RFC][PATCH 5/7] cgroup: make sure parent won't be destroyed before its children

[Li Zefan <lizefan@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 due
to css ref draining to 0. Now it would be a disaster if the child cgroup
tries to access its parent.

Make sure this won't happen.

Signed-off-by: Li Zefan <lizefan@huawei.com>
---
 kernel/cgroup.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index fa54b92..78204bc 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -888,6 +888,13 @@ static void cgroup_free_fn(struct work_struct *work)
 	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
 	 */
@@ -4171,6 +4178,9 @@ 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);
-- 
1.8.0.2

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