[PATCH] cgroup: remove unnecessary empty check when enabling threaded mode

[Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

cgroup_enable_threaded() checks that the cgroup doesn't have any tasks
or children and fails the operation if so.  This test is unnecessary
because the first part is already checked by
cgroup_can_be_thread_root() and the latter is unnecessary.  The latter
actually cause a behavioral oddity.  Please consider the following
hierarchy.  All cgroups are domains.

    A
   / \
  B   C
       \
        D

If B is made threaded, C and D becomes invalid domains.  Due to the no
children restriction, threaded mode can't be enabled on C.  For C and
D, the only thing the user can do is removal.

There is no reason for this restriction.  Remove it.

Signed-off-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Cc: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
---
Hello,

So, the only thing inconsistent there was the extra restriction when
enabling threaded mode when all the necessary conditions are already
checked by when verifying the parent's domain.

Thanks.

 Documentation/cgroup-v2.txt |    5 +++--
 kernel/cgroup/cgroup.c      |    7 -------
 2 files changed, 3 insertions(+), 9 deletions(-)

--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -274,8 +274,9 @@ thread mode, the following conditions mu
 - As the cgroup will join the parent's resource domain.  The parent
   must either be a valid (threaded) domain or a threaded cgroup.
 
-- The cgroup must be empty.  No enabled controllers, child cgroups or
-  processes.
+- When the parent is an unthreaded domain, it must not have any domain
+  controllers enabled or populated domain children.  The root is
+  exempt from this requirement.
 
 Topology-wise, a cgroup can be in an invalid state.  Please consider
 the following toplogy::
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3147,13 +3147,6 @@ static int cgroup_enable_threaded(struct
 		return -EOPNOTSUPP;
 
 	/*
-	 * Allow enabling thread mode only on empty cgroups to avoid
-	 * implicit migrations and recursive operations.
-	 */
-	if (cgroup_has_tasks(cgrp) || css_has_online_children(&cgrp->self))
-		return -EBUSY;
-
-	/*
 	 * The following shouldn't cause actual migrations and should
 	 * always succeed.
 	 */