cgroup: introduce task_css_is_root()

Determining the css of a task usually requires RCU read lock as that's
the only thing which keeps the returned css accessible till its
reference is acquired; however, testing whether a task belongs to the
root can be performed without dereferencing the returned css by
comparing the returned pointer against the root one in init_css_set[]
which never changes.

Implement task_css_is_root() which can be invoked in any context.
This will be used by the scheduled cgroup_freezer change.

v2: cgroup no longer supports modular controllers.  No need to export
    init_css_set.  Pointed out by Li.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 11a03d67635a..3f1ca934a237 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -348,7 +348,7 @@ struct cgrp_cset_link {
  * reference-counted, to improve performance when child cgroups
  * haven't been created.
  */
-static struct css_set init_css_set = {
+struct css_set init_css_set = {
         .refcount               = ATOMIC_INIT(1),
         .cgrp_links             = LIST_HEAD_INIT(init_css_set.cgrp_links),
         .tasks                  = LIST_HEAD_INIT(init_css_set.tasks),