netfilter: nf_log: don't call synchronize_rcu in nf_log_unset

nf_log_unregister() (which is what gets called in the logger backends
module exit paths) does a (required, module is removed) synchronize_rcu().

But nf_log_unset() is only called from pernet exit handlers. It doesn't
free any memory so there appears to be no need to call synchronize_rcu.

v2: Liping Zhang points out that nf_log_unregister() needs to be called
after pernet unregister, else rmmod would become unsafe.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_log.c

@@ -71,7 +71,6 @@ void nf_log_unset(struct net *net, const struct nf_logger *logger)
 			RCU_INIT_POINTER(net->nf.nf_loggers[i], NULL);
 	}
 	mutex_unlock(&nf_log_mutex);
-	synchronize_rcu();
 }
 EXPORT_SYMBOL(nf_log_unset);
 

net/netfilter/nfnetlink_log.c

@@ -1140,10 +1140,10 @@ out:
 
 static void __exit nfnetlink_log_fini(void)
 {
-	nf_log_unregister(&nfulnl_logger);
 	nfnetlink_subsys_unregister(&nfulnl_subsys);
 	netlink_unregister_notifier(&nfulnl_rtnl_notifier);
 	unregister_pernet_subsys(&nfnl_log_net_ops);
+	nf_log_unregister(&nfulnl_logger);
 }
 
 MODULE_DESCRIPTION("netfilter userspace logging");