cgroup: create dfl_root files on subsys registration

On subsystem registration, css_populate_dir() is not called on the new
root css, so the interface files for the subsystem on cgrp_dfl_root
aren't created on registration.  This is a residue from the days when
cgrp_dfl_root was used only as the parking spot for unused subsystems,
which no longer is true as it's used as the root for cgroup2.

This is often fine as later operations tend to create them as a part
of mount (cgroup1) or subtree_control operations (cgroup2); however,
it's not difficult to mount cgroup2 with the controller interface
files missing as Waiman found out.

Fix it by invoking css_populate_dir() on the root css on subsys
registration.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-and-tested-by: Waiman Long <longman@redhat.com>
Cc: stable@vger.kernel.org # v4.5+
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index cc53111072d84e873a873727f51034b523b79c4e..744975947d01b2b865927fee8ec6cbec5aa034c8 100644 (file)
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4673,6 +4673,10 @@ int __init cgroup_init(void)
 
                if (ss->bind)
                        ss->bind(init_css_set.subsys[ssid]);
+
+               mutex_lock(&cgroup_mutex);
+               css_populate_dir(init_css_set.subsys[ssid]);
+               mutex_unlock(&cgroup_mutex);
        }
 
        /* init_css_set.subsys[] has been updated, re-hash */