[PATCH net v2] net: Do not hold the reference for the same sk_rx_dst

In some rare cases, inet_sk_rx_dst_set() may be called multiple times
on the same dst, causing double refcounting. Eventually, it
prevents net_device to be destroyed. The bug manifested as

unregister_netdevice: waiting for lo to become free. Usage count = 1

in the kernel log, preventing new network namespace creation.

Signed-off-by: Kevin Xu <redacted>
---
 net/ipv4/tcp_ipv4.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 575e19d..ca588d1 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c

@@ -1806,9 +1806,11 @@ int tcp_v4_rcv(struct sk_buff *skb)
 void inet_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb)
 {
 	struct dst_entry *dst = skb_dst(skb);
+	struct dst_entry *old;
 
 	if (dst && dst_hold_safe(dst)) {
-		sk->sk_rx_dst = dst;
+		old = xchg(&sk->sk_rx_dst, dst);
+		dst_release(old);
 		inet_sk(sk)->rx_dst_ifindex = skb->skb_iif;
 	}
 }

-- 
1.9.1