From: Eric Dumazet <edumazet@google.com>
Date: Wed, 2 Mar 2022 20:05:13 +0100
Subject: batman-adv: make mc_forwarding atomic

This fixes minor data-races in ip6_mc_input() and
batadv_mcast_mla_rtr_flags_softif_get_ipv6()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[sven@narfation.org: Add ugly hack to get it building with old kernels]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Origin: upstream, https://git.open-mesh.org/batman-adv.git/commit/56db7c0540e733a1f063ccd6bab1b537a80857eb

--- a/net/batman-adv/multicast.c
+++ b/net/batman-adv/multicast.c
@@ -134,7 +134,11 @@ static u8 batadv_mcast_mla_rtr_flags_sof
 {
 	struct inet6_dev *in6_dev = __in6_dev_get(dev);
 
+#if LINUX_VERSION_IS_GEQ(5, 18, 0) // UGLY_HACK_NEW
+	if (in6_dev && atomic_read(&in6_dev->cnf.mc_forwarding))
+#else // UGLY_HACK_OLD
 	if (in6_dev && in6_dev->cnf.mc_forwarding)
+#endif // UGLY_HACK_STOP
 		return BATADV_NO_FLAGS;
 	else
 		return BATADV_MCAST_WANT_NO_RTR6;