cgroup: move cgroup->sibling unlinking to cgroup_put()

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>

kernel/cgroup.c

@@ -1056,6 +1056,11 @@ static void cgroup_put(struct cgroup *cgrp)
 	if (WARN_ON_ONCE(cgrp->parent && !cgroup_is_dead(cgrp)))
 		return;
 
+	/* delete this cgroup from parent->children */
+	mutex_lock(&cgroup_mutex);
+	list_del_rcu(&cgrp->sibling);
+	mutex_unlock(&cgroup_mutex);
+
 	cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
 	cgrp->id = -1;
 
@@ -4561,9 +4566,6 @@ static void cgroup_destroy_css_killed(struct cgroup *cgrp)
 {
 	lockdep_assert_held(&cgroup_mutex);
 
-	/* delete this cgroup from parent->children */
-	list_del_rcu(&cgrp->sibling);
-
 	cgroup_put(cgrp);
 }