tipc: fix nametbl_lock soft lockup at node/link events

We trigger a soft lockup as we grab nametbl_lock twice if the node
has a pending node up/down or link up/down event while:
- we process an incoming named message in tipc_named_rcv() and
  perform an tipc_update_nametbl().
- we have pending backlog items in the name distributor queue
  during a nametable update using tipc_nametbl_publish() or
  tipc_nametbl_withdraw().

The following are the call chain associated:
tipc_named_rcv() Grabs nametbl_lock
   tipc_update_nametbl() (publish/withdraw)
     tipc_node_subscribe()/unsubscribe()
       tipc_node_write_unlock()
          << lockup occurs if an outstanding node/link event
             exits, as we grabs nametbl_lock again >>

tipc_nametbl_withdraw() Grab nametbl_lock
  tipc_named_process_backlog()
    tipc_update_nametbl()
      << rest as above >>

The function tipc_node_write_unlock(), in addition to releasing the
lock processes the outstanding node/link up/down events. To do this,
we need to grab the nametbl_lock again leading to the lockup.

In this commit we fix the soft lockup by introducing a fast variant of
node_unlock(), where we just release the lock. We adapt the
node_subscribe()/node_unsubscribe() to use the fast variants.

Reported-and-Tested-by: John Thompson <thompa.atl@gmail.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Parthasarathy Bhuvaragan <parthasarathy.bhuvaragan@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/node.c

@@ -263,6 +263,11 @@ static void tipc_node_write_lock(struct tipc_node *n)
 	write_lock_bh(&n->lock);
 }
 
+static void tipc_node_write_unlock_fast(struct tipc_node *n)
+{
+	write_unlock_bh(&n->lock);
+}
+
 static void tipc_node_write_unlock(struct tipc_node *n)
 {
 	struct net *net = n->net;
@@ -417,7 +422,7 @@ void tipc_node_subscribe(struct net *net, struct list_head *subscr, u32 addr)
 	}
 	tipc_node_write_lock(n);
 	list_add_tail(subscr, &n->publ_list);
-	tipc_node_write_unlock(n);
+	tipc_node_write_unlock_fast(n);
 	tipc_node_put(n);
 }
 
@@ -435,7 +440,7 @@ void tipc_node_unsubscribe(struct net *net, struct list_head *subscr, u32 addr)
 	}
 	tipc_node_write_lock(n);
 	list_del_init(subscr);
-	tipc_node_write_unlock(n);
+	tipc_node_write_unlock_fast(n);
 	tipc_node_put(n);
 }