[PATCH v2 net] net/sched: make psched_mtu() RTNL-less safe

From: Pedro Tammela <pctammela@mojatatu.com>
To: netdev@vger.kernel.org
Cc: jhs@mojatatu.com, xiyou.wangcong@gmail.com, jiri@resnulli.us,
	davem@davemloft.net, edumazet@google.com, kuba@kernel.org,
	pabeni@redhat.com, Pedro Tammela <pctammela@mojatatu.com>
Subject: [PATCH v2 net] net/sched: make psched_mtu() RTNL-less safe
Date: Mon, 10 Jul 2023 23:16:34 -0300	[thread overview]
Message-ID: <20230711021634.561598-1-pctammela@mojatatu.com> (raw)

Eric Dumazet says[1]:
-------
Speaking of psched_mtu(), I see that net/sched/sch_pie.c is using it
without holding RTNL, so dev->mtu can be changed underneath.
KCSAN could issue a warning.
-------

Annotate dev->mtu with READ_ONCE() so KCSAN don't issue a warning.

[1] https://lore.kernel.org/all/CANn89iJoJO5VtaJ-2=_d2aOQhb0Xw8iBT_Cxqp2HyuS-zj6azw@mail.gmail.com/

v1 -> v2: Fix commit message

Fixes: d4b36210c2e6 ("net: pkt_sched: PIE AQM scheme")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
---
 include/net/pkt_sched.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h
index e98aac9d5ad5..15960564e0c3 100644
--- a/include/net/pkt_sched.h
+++ b/include/net/pkt_sched.h
@@ -134,7 +134,7 @@ extern const struct nla_policy rtm_tca_policy[TCA_MAX + 1];
  */
 static inline unsigned int psched_mtu(const struct net_device *dev)
 {
-	return dev->mtu + dev->hard_header_len;
+	return READ_ONCE(dev->mtu) + dev->hard_header_len;
 }
 
 static inline struct net *qdisc_net(struct Qdisc *q)
-- 
2.39.2

