memcg: let css_get_next() rely upon rcu_read_lock()

Remove lock and unlock around css_get_next()'s call to idr_get_next().
memcg iterators (only users of css_get_next) already did rcu_read_lock(),
and its comment demands that; but add a WARN_ON_ONCE to make sure of it.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 3 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 8eb90f25bd7b..391d5e991e5f 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -5033,6 +5033,8 @@ css_get_next(struct cgroup_subsys *ss, int id,
                 return NULL;
 
         BUG_ON(!ss->use_id);
+        WARN_ON_ONCE(!rcu_read_lock_held());
+
         /* fill start point for scan */
         tmpid = id;
         while (1) {
@@ -5040,10 +5042,7 @@ css_get_next(struct cgroup_subsys *ss, int id,
                  * scan next entry from bitmap(tree), tmpid is updated after
                  * idr_get_next().
                  */
-                spin_lock(&ss->id_lock);
                 tmp = idr_get_next(&ss->idr, &tmpid);
-                spin_unlock(&ss->id_lock);
-
                 if (!tmp)
                         break;
                 if (tmp->depth >= depth && tmp->stack[depth] == rootid) {