Move cgroup->sibling unlinking from cgroup_destroy_css_killed() to
cgroup_put(). This is later but still before the RCU grace period, so
it doesn't break css_next_child() although there now is a larger
window in which a dead cgroup is visible during css iteration. As css
iteration always could have included offline csses, this doesn't
affect correctness; however, it does make css_next_child() fall back
to reiterting mode more often. This also makes cgroup_put() directly
take cgroup_mutex, which limits where it can be called from. These
are not immediately problematic and will be dealt with later.
This change enables simplification of cgroup destruction path.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
4e4e284723