cls_cgroup: read classid atomically in classifier

Avoid reading the unsynchronized value cs->classid multiple times,
since it could change concurrently from non-zero to zero; this would
result in the classifier returning a positive result with a bogus
(zero) classid.

Signed-off-by: Paul Menage <menage@google.com>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/sched/cls_cgroup.c b/net/sched/cls_cgroup.c
index 91a3db4..cc29b44 100644 (file)
--- a/net/sched/cls_cgroup.c
+++ b/net/sched/cls_cgroup.c
@@ -104,8 +104,7 @@ static int cls_cgroup_classify(struct sk_buff *skb, struct tcf_proto *tp,
                               struct tcf_result *res)
 {
        struct cls_cgroup_head *head = tp->root;
-       struct cgroup_cls_state *cs;
-       int ret = 0;
+       u32 classid;
 
        /*
         * Due to the nature of the classifier it is required to ignore all
@@ -121,17 +120,18 @@ static int cls_cgroup_classify(struct sk_buff *skb, struct tcf_proto *tp,
                return -1;
 
        rcu_read_lock();
-       cs = task_cls_state(current);
-       if (cs->classid && tcf_em_tree_match(skb, &head->ematches, NULL)) {
-               res->classid = cs->classid;
-               res->class = 0;
-               ret = tcf_exts_exec(skb, &head->exts, res);
-       } else
-               ret = -1;
-
+       classid = task_cls_state(current)->classid;
        rcu_read_unlock();
 
-       return ret;
+       if (!classid)
+               return -1;
+
+       if (!tcf_em_tree_match(skb, &head->ematches, NULL))
+               return -1;
+
+       res->classid = classid;
+       res->class = 0;
+       return tcf_exts_exec(skb, &head->exts, res);
 }
 
 static unsigned long cls_cgroup_get(struct tcf_proto *tp, u32 handle)