cgroup: update subsystem rebind restrictions

Because the default root couldn't have any non-root csses attached to
it, rebinding away from it was always allowed; however, the default
hierarchy will soon host the unified hierarchy and have non-root csses
so the rebind restrictions need to be updated accordingly.

Instead of special casing rebinding from the default hierarchy and
then checking whether the source hierarchy has children cgroups, which
implies non-root csses for !dfl hierarchies, simply check whether the
source hierarchy has non-root csses for the subsystem using
css_next_child().

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 c66bfc8ee8a7227d0120c697a6e3ed7dede0e556..15eb2273d80b284e591b2b4e0295d9b6ee35816b 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1051,16 +1051,12 @@ static int rebind_subsystems(struct cgroup_root *dst_root,
                if (!(ss_mask & (1 << ssid)))
                        continue;
 
-               /* if @ss is on the dummy_root, we can always move it */
-               if (ss->root == &cgrp_dfl_root)
-                       continue;
-
-               /* if @ss has non-root cgroups attached to it, can't move */
-               if (!list_empty(&ss->root->cgrp.children))
+               /* if @ss has non-root csses attached to it, can't move */
+               if (css_next_child(NULL, cgroup_css(&ss->root->cgrp, ss)))
                        return -EBUSY;
 
                /* can't move between two non-dummy roots either */
-               if (dst_root != &cgrp_dfl_root)
+               if (ss->root != &cgrp_dfl_root && dst_root != &cgrp_dfl_root)
                        return -EBUSY;
        }