Commit 0cee8b77 authored May 13, 2014 by Tejun Heo

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>

parent f21a4f75

kernel/cgroup.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -2594,16 +2594,18 @@ static int cgroup_subtree_control_write(struct cgroup_subsys_state *dummy_css,
		* 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;
		}