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>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 9db1a9629a5c3eb7d5ec9d3bfb78ab867f370112..95fc66b16091dfc2c335eb4de0b3564c961d46d1 100644 (file)
--- 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;
                        }