From: Eric Dumazet <edumazet@google.com>
Date: Thu, 28 Aug 2025 19:58:16 +0000 (+0000)
Subject: net: dst: introduce dst->dev_rcu
X-Git-Tag: v6.18-rc1~132^2~295^2~7
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=caedcc5b6df1b2e2b5f39079e3369c1d4d5c5f50;p=thirdparty%2Fkernel%2Flinux.git

net: dst: introduce dst->dev_rcu

Followup of commit 88fe14253e18 ("net: dst: add four helpers
to annotate data-races around dst->dev").

We want to gradually add explicit RCU protection to dst->dev,
including lockdep support.

Add an union to alias dst->dev_rcu and dst->dev.

Add dst_dev_net_rcu() helper.

Fixes: 4a6ce2b6f2ec ("net: introduce a new function dst_dev_put()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250828195823.3958522-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---

diff --git a/include/net/dst.h b/include/net/dst.h
index bab01363bb975..f8aa1239b4db6 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -24,7 +24,10 @@
 struct sk_buff;
 
 struct dst_entry {
-	struct net_device       *dev;
+	union {
+		struct net_device       *dev;
+		struct net_device __rcu *dev_rcu;
+	};
 	struct  dst_ops	        *ops;
 	unsigned long		_metrics;
 	unsigned long           expires;
@@ -570,9 +573,12 @@ static inline struct net_device *dst_dev(const struct dst_entry *dst)
 
 static inline struct net_device *dst_dev_rcu(const struct dst_entry *dst)
 {
-	/* In the future, use rcu_dereference(dst->dev) */
-	WARN_ON_ONCE(!rcu_read_lock_held());
-	return READ_ONCE(dst->dev);
+	return rcu_dereference(dst->dev_rcu);
+}
+
+static inline struct net *dst_dev_net_rcu(const struct dst_entry *dst)
+{
+	return dev_net_rcu(dst_dev_rcu(dst));
 }
 
 static inline struct net_device *skb_dst_dev(const struct sk_buff *skb)
@@ -592,7 +598,7 @@ static inline struct net *skb_dst_dev_net(const struct sk_buff *skb)
 
 static inline struct net *skb_dst_dev_net_rcu(const struct sk_buff *skb)
 {
-	return dev_net_rcu(skb_dst_dev(skb));
+	return dev_net_rcu(skb_dst_dev_rcu(skb));
 }
 
 struct dst_entry *dst_blackhole_check(struct dst_entry *dst, u32 cookie);
diff --git a/net/core/dst.c b/net/core/dst.c
index e2de8b68c41d3..e9d35f49c9e78 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -150,7 +150,7 @@ void dst_dev_put(struct dst_entry *dst)
 		dst->ops->ifdown(dst, dev);
 	WRITE_ONCE(dst->input, dst_discard);
 	WRITE_ONCE(dst->output, dst_discard_out);
-	WRITE_ONCE(dst->dev, blackhole_netdev);
+	rcu_assign_pointer(dst->dev_rcu, blackhole_netdev);
 	netdev_ref_replace(dev, blackhole_netdev, &dst->dev_tracker,
 			   GFP_ATOMIC);
 }
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index cc86a917a1bbb..44382d1755890 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1027,7 +1027,7 @@ static void __ip_rt_update_pmtu(struct rtable *rt, struct flowi4 *fl4, u32 mtu)
 		return;
 
 	rcu_read_lock();
-	net = dev_net_rcu(dst_dev(dst));
+	net = dst_dev_net_rcu(dst);
 	if (mtu < net->ipv4.ip_rt_min_pmtu) {
 		lock = true;
 		mtu = min(old_mtu, net->ipv4.ip_rt_min_pmtu);
@@ -1327,7 +1327,7 @@ static unsigned int ipv4_default_advmss(const struct dst_entry *dst)
 	struct net *net;
 
 	rcu_read_lock();
-	net = dev_net_rcu(dst_dev(dst));
+	net = dst_dev_net_rcu(dst);
 	advmss = max_t(unsigned int, ipv4_mtu(dst) - header_size,
 				   net->ipv4.ip_rt_min_advmss);
 	rcu_read_unlock();