Commit 918a8c2c authored by Tejun Heo's avatar Tejun Heo

cgroup: remove unnecessary empty check when enabling threaded mode

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.
Acked-by: default avatarWaiman Long <longman@redhat.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 7a0cf0e7
......@@ -274,8 +274,9 @@ thread mode, the following conditions must be met.
- 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::
......
......@@ -3146,13 +3146,6 @@ static int cgroup_enable_threaded(struct cgroup *cgrp)
!cgroup_can_be_thread_root(dom_cgrp))
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.
......
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