[RFC][PATCH 8/9 v2] cgroup: avoid creating new cgroup under a cgroup being destroyed

[KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>]

When ->pre_destroy() is called, it should be guaranteed that
new child cgroup is not created under a cgroup, where pre_destroy()
is running. If not, ->pre_destroy() must check children and
return -EBUSY, which causes warning.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
---
 kernel/cgroup.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 7a3076b..003ceed 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3970,6 +3970,15 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 
 	mutex_lock(&cgroup_mutex);
 
+	/*
+	 * prevent making child cgroup when ->pre_destroy() is running.
+	 */
+	if (test_bit(CGRP_WAIT_ON_RMDIR, &parent->flags)) {
+		mutex_unlock(&cgroup_mutex);
+		atomic_dec(&sb->s_active);
+		return -EBUSY;
+	}
+
 	init_cgroup_housekeeping(cgrp);
 
 	cgrp->parent = parent;
-- 
1.7.4.1