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>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 8eb90f2..391d5e9 100644 (file)
--- 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) {