cgroup: fix offlining child waiting in cgroup_subtree_control_write()

cgroup_subtree_control_write() waits for offline to complete
child-by-child before enabling a controller; however, it has a couple
bugs.

* It doesn't initialize the wait_queue_t.  This can lead to infinite
  hang on the following schedule() among other things.

* It forgets to pin the child before releasing cgroup_tree_mutex and
  performing schedule().  The child may already be gone by the time it
  wakes up and invokes finish_wait().  Pin the child being waited on.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 9db1a9629a5c..95fc66b16091 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2594,16 +2594,18 @@ retry:
                          * cases, wait till it's gone using offline_waitq.
                          */
                         cgroup_for_each_live_child(child, cgrp) {
-                                wait_queue_t wait;
+                                DEFINE_WAIT(wait);
 
                                 if (!cgroup_css(child, ss))
                                         continue;
 
+                                cgroup_get(child);
                                 prepare_to_wait(&child->offline_waitq, &wait,
                                                 TASK_UNINTERRUPTIBLE);
                                 mutex_unlock(&cgroup_tree_mutex);
                                 schedule();
                                 finish_wait(&child->offline_waitq, &wait);
+                                cgroup_put(child);
                                 goto retry;
                         }