[PATCH 3/3] memcg: let css_get_next() rely upon rcu_read_lock()

[Hugh Dickins <hughd@google.com>]

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>
---
 kernel/cgroup.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- 3.2.0+.orig/kernel/cgroup.c	2012-01-19 12:16:04.000000000 -0800
+++ 3.2.0+/kernel/cgroup.c	2012-01-19 12:22:55.188244820 -0800
@@ -5087,6 +5087,8 @@ css_get_next(struct cgroup_subsys *ss, i
 		return NULL;
 
 	BUG_ON(!ss->use_id);
+	WARN_ON_ONCE(!rcu_read_lock_held());
+
 	/* fill start point for scan */
 	tmpid = id;
 	while (1) {
@@ -5094,10 +5096,7 @@ css_get_next(struct cgroup_subsys *ss, i
 		 * 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) {

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