net: tipc: remove one synchronize_net() from tipc_nametbl_stop()

tipc_exit_net() is very slow and is abused by syzbot.

tipc_nametbl_stop() is called for each netns being dismantled.

Calling synchronize_net() right before freeing tn->nametbl
is a big hammer.

Replace this with kfree_rcu().

Note that RCU is not properly used here, otherwise
tn->nametbl should be cleared before the synchronize_net()
or kfree_rcu(), or even before the cleanup loop.

We might need to fix this at some point.

Also note tipc uses other synchronize_rcu() calls,
more work is needed to make tipc_exit_net() much faster.

List of remaining calls to synchronize_rcu()

  tipc_detach_loopback() (dev_remove_pack())
  tipc_bcast_stop()
  tipc_sk_rht_destroy()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241204210234.319484-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/tipc/name_table.c

@@ -949,8 +949,8 @@ void tipc_nametbl_stop(struct net *net)
 	}
 	spin_unlock_bh(&tn->nametbl_lock);
 
-	synchronize_net();
-	kfree(nt);
+	/* TODO: clear tn->nametbl, implement proper RCU rules ? */
+	kfree_rcu(nt, rcu);
 }
 
 static int __tipc_nl_add_nametable_publ(struct tipc_nl_msg *msg,

net/tipc/name_table.h

@@ -90,6 +90,7 @@ struct publication {
 
 /**
  * struct name_table - table containing all existing port name publications
+ * @rcu: RCU callback head used for deferred freeing
  * @services: name sequence hash lists
  * @node_scope: all local publications with node scope
  *               - used by name_distr during re-init of name table
@@ -102,6 +103,7 @@ struct publication {
  * @snd_nxt: next sequence number to be used
  */
 struct name_table {
+	struct rcu_head rcu;
 	struct hlist_head services[TIPC_NAMETBL_SIZE];
 	struct list_head node_scope;
 	struct list_head cluster_scope;