tipc: adjust locking policy of subscription

Currently subscriber's lock protects not only subscriber's subscription
list but also all subscriptions linked into the list. However, as all
members of subscription are never changed after they are initialized,
it's unnecessary for subscription to be protected under subscriber's
lock. If the lock is used to only protect subscriber's subscription
list, the adjustment not only makes the locking policy simpler, but
also helps to avoid a deadlock which may happen once creating a
subscription is failed.

Signed-off-by: Ying Xue <[e-mail]>
Reviewed-by: Jon Maloy <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/tipc/subscr.c

···
 {
 	struct tipc_subscription *sub, *temp;
 
+	spin_lock_bh(&subscriber->lock);
 	/* Find first matching subscription, exit if not found */
 	list_for_each_entry_safe(sub, temp, &subscriber->subscrp_list,
 				 subscrp_list) {
···
 			break;
 		}
 	}
+	spin_unlock_bh(&subscriber->lock);
 }
 
 static int tipc_subscrp_create(struct net *net, struct tipc_subscr *s,
···
 		kfree(sub);
 		return -EINVAL;
 	}
+	spin_lock_bh(&subscriber->lock);
 	list_add(&sub->subscrp_list, &subscriber->subscrp_list);
+	spin_unlock_bh(&subscriber->lock);
 	sub->subscriber = subscriber;
 	sub->swap = swap;
 	memcpy(&sub->evt.s, s, sizeof(*s));
···
 	struct tipc_subscription *sub = NULL;
 	struct tipc_net *tn = net_generic(net, tipc_net_id);
 
-	spin_lock_bh(&subscriber->lock);
 	tipc_subscrp_create(net, (struct tipc_subscr *)buf, subscriber, &sub);
 	if (sub)
 		tipc_nametbl_subscribe(sub);
 	else
 		tipc_conn_terminate(tn->topsrv, subscriber->conid);
-	spin_unlock_bh(&subscriber->lock);
 }
 
 /* Handle one request to establish a new subscriber */