netdev - [PATCH] cls_cgroup: read classid atomically in classifier

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Message-ID: <20090521183100.12678.28835.stgit@menage.mtv.corp.google.com>
Date:	Thu, 21 May 2009 11:31:26 -0700
From:	Paul Menage <menage@...gle.com>
To:	lizf@...fujitsu.com, akpm@...ux-foundation.org,
	davem@...emloft.net, tgraf@...g.ch
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: [PATCH] cls_cgroup: read classid atomically in classifier

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@...gle.com>
Reviewed-by: Li Zefan <lizf@...fujitsu.com>

---

Resending to cc netdev@...r.kernel.org as requested by DaveM

 net/sched/cls_cgroup.c |   22 +++++++++++-----------
 1 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/net/sched/cls_cgroup.c b/net/sched/cls_cgroup.c
index 1ab4542..4ece6e0 100644
--- a/net/sched/cls_cgroup.c
+++ b/net/sched/cls_cgroup.c
@@ -98,8 +98,7 @@ 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
@@ -115,17 +114,18 @@ 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)

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