net: Allow xfrm_user_net_exit to batch efficiently.

xfrm.nlsk is provided by the xfrm_user module and is access via rcu from
other parts of the xfrm code.  Add xfrm.nlsk_stash a copy of xfrm.nlsk that
will never be set to NULL.  This allows the synchronize_net and
netlink_kernel_release to be deferred until a whole batch of xfrm.nlsk sockets
have been set to NULL.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/xfrm.h

@@ -43,6 +43,7 @@ struct netns_xfrm {
 	struct work_struct	policy_hash_work;
 
 	struct sock		*nlsk;
+	struct sock		*nlsk_stash;
 
 	u32			sysctl_aevent_etime;
 	u32			sysctl_aevent_rseqth;

net/xfrm/xfrm_user.c

@@ -2721,22 +2721,24 @@ static int __net_init xfrm_user_net_init(struct net *net)
 				     xfrm_netlink_rcv, NULL, THIS_MODULE);
 	if (nlsk == NULL)
 		return -ENOMEM;
+	net->xfrm.nlsk_stash = nlsk; /* Don't set to NULL */
 	rcu_assign_pointer(net->xfrm.nlsk, nlsk);
 	return 0;
 }
 
-static void __net_exit xfrm_user_net_exit(struct net *net)
+static void __net_exit xfrm_user_net_exit(struct list_head *net_exit_list)
 {
-	struct sock *nlsk = net->xfrm.nlsk;
-
+	struct net *net;
+	list_for_each_entry(net, net_exit_list, exit_list)
 		rcu_assign_pointer(net->xfrm.nlsk, NULL);
-	synchronize_rcu();
-	netlink_kernel_release(nlsk);
+	synchronize_net();
+	list_for_each_entry(net, net_exit_list, exit_list)
+		netlink_kernel_release(net->xfrm.nlsk_stash);
 }
 
 static struct pernet_operations xfrm_user_net_ops = {
 	.init	    = xfrm_user_net_init,
-	.exit = xfrm_user_net_exit,
+	.exit_batch = xfrm_user_net_exit,
 };
 
 static int __init xfrm_user_init(void)