net: initialize net->net_cookie at netns setup

It is simpler to make net->net_cookie a plain u64
written once in setup_net() instead of looping
and using atomic64 helpers.

Lorenz Bauer wants to add SO_NETNS_COOKIE socket option
and this patch would makes his patch series simpler.

Signed-off-by: Eric Dumazet <[e-mail]>
Cc: Daniel Borkmann <[e-mail]>
Cc: Lorenz Bauer <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Tested-by: Lorenz Bauer <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/net_namespace.h

···
 	struct netns_xfrm	xfrm;
 #endif
 
-	atomic64_t		net_cookie; /* written once */
+	u64			net_cookie; /* written once */
 
 #if IS_ENABLED(CONFIG_IP_VS)
 	struct netns_ipvs	*ipvs;
···
 
 struct net *get_net_ns_by_pid(pid_t pid);
 struct net *get_net_ns_by_fd(int fd);
-
-u64 __net_gen_cookie(struct net *net);
 
 #ifdef CONFIG_SYSCTL
 void ipx_register_sysctl(void);

net/core/filter.c

···
 
 static u64 __bpf_get_netns_cookie(struct sock *sk)
 {
-#ifdef CONFIG_NET_NS
-	return __net_gen_cookie(sk ? sk->sk_net.net : &init_net);
-#else
-	return 0;
-#endif
+	const struct net *net = sk ? sock_net(sk) : &init_net;
+
+	return net->net_cookie;
 }
 
 BPF_CALL_1(bpf_get_netns_cookie_sock, struct sock *, ctx)

net/core/net_namespace.c

···
 
 DEFINE_COOKIE(net_cookie);
 
-u64 __net_gen_cookie(struct net *net)
-{
-	while (1) {
-		u64 res = atomic64_read(&net->net_cookie);
-
-		if (res)
-			return res;
-		res = gen_cookie_next(&net_cookie);
-		atomic64_cmpxchg(&net->net_cookie, 0, res);
-	}
-}
-
 static struct net_generic *net_alloc_generic(void)
 {
 	struct net_generic *ng;
···
 	refcount_set(&net->ns.count, 1);
 	refcount_set(&net->passive, 1);
 	get_random_bytes(&net->hash_mix, sizeof(u32));
+	preempt_disable();
+	net->net_cookie = gen_cookie_next(&net_cookie);
+	preempt_enable();
 	net->dev_base_seq = 1;
 	net->user_ns = user_ns;
 	idr_init(&net->netns_ids);
···
 		panic("Could not allocate generic netns");
 
 	rcu_assign_pointer(init_net.gen, ng);
-
-	preempt_disable();
-	__net_gen_cookie(&init_net);
-	preempt_enable();
 
 	down_write(&pernet_ops_rwsem);
 	if (setup_net(&init_net, &init_user_ns))