netfilter: nf_dup_netdev: Move the recursion counter struct netdev_xmit

nf_dup_skb_recursion is a per-CPU variable and relies on disabled BH for its
locking. Without per-CPU locking in local_bh_disable() on PREEMPT_RT
this data structure requires explicit locking.

Move nf_dup_skb_recursion to struct netdev_xmit, provide wrappers.

Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/linux/netdevice_xmit.h

···
 #if IS_ENABLED(CONFIG_NET_ACT_MIRRED)
 	u8 sched_mirred_nest;
 #endif
+#if IS_ENABLED(CONFIG_NF_DUP_NETDEV)
+	u8 nf_dup_skb_recursion;
+#endif
 };
 
 #endif

net/netfilter/nf_dup_netdev.c

···
 
 #define NF_RECURSION_LIMIT	2
 
-static DEFINE_PER_CPU(u8, nf_dup_skb_recursion);
+#ifndef CONFIG_PREEMPT_RT
+static u8 *nf_get_nf_dup_skb_recursion(void)
+{
+	return this_cpu_ptr(&softnet_data.xmit.nf_dup_skb_recursion);
+}
+#else
+
+static u8 *nf_get_nf_dup_skb_recursion(void)
+{
+	return &current->net_xmit.nf_dup_skb_recursion;
+}
+
+#endif
 
 static void nf_do_netdev_egress(struct sk_buff *skb, struct net_device *dev,
 				enum nf_dev_hooks hook)
 {
-	if (__this_cpu_read(nf_dup_skb_recursion) > NF_RECURSION_LIMIT)
+	u8 *nf_dup_skb_recursion = nf_get_nf_dup_skb_recursion();
+
+	if (*nf_dup_skb_recursion > NF_RECURSION_LIMIT)
 		goto err;
 
 	if (hook == NF_NETDEV_INGRESS && skb_mac_header_was_set(skb)) {
···
 
 	skb->dev = dev;
 	skb_clear_tstamp(skb);
-	__this_cpu_inc(nf_dup_skb_recursion);
+	(*nf_dup_skb_recursion)++;
 	dev_queue_xmit(skb);
-	__this_cpu_dec(nf_dup_skb_recursion);
+	(*nf_dup_skb_recursion)--;
 	return;
 err:
 	kfree_skb(skb);