net: Revert "ipv4: get rid of ip_ra_lock"

This reverts commit ba3f571d5dde. The commit was made
after 1215e51edad1 "ipv4: fix a deadlock in ip_ra_control",
and killed ip_ra_lock, which became useless after rtnl_lock()
made used to destroy every raw ipv4 socket. This scales
very bad, and next patch in series reverts 1215e51edad1.
ip_ra_lock will be used again.

Signed-off-by: Kirill Tkhai <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/ip_sockglue.c

···
    sent to multicast group to reach destination designated router.
  */
 struct ip_ra_chain __rcu *ip_ra_chain;
+static DEFINE_SPINLOCK(ip_ra_lock);
 
 
 static void ip_ra_destroy_rcu(struct rcu_head *head)
···
 
 	new_ra = on ? kmalloc(sizeof(*new_ra), GFP_KERNEL) : NULL;
 
+	spin_lock_bh(&ip_ra_lock);
 	for (rap = &ip_ra_chain;
-	     (ra = rtnl_dereference(*rap)) != NULL;
+	     (ra = rcu_dereference_protected(*rap,
+			lockdep_is_held(&ip_ra_lock))) != NULL;
 	     rap = &ra->next) {
 		if (ra->sk == sk) {
 			if (on) {
+				spin_unlock_bh(&ip_ra_lock);
 				kfree(new_ra);
 				return -EADDRINUSE;
 			}
 			/* dont let ip_call_ra_chain() use sk again */
 			ra->sk = NULL;
 			RCU_INIT_POINTER(*rap, ra->next);
+			spin_unlock_bh(&ip_ra_lock);
 
 			if (ra->destructor)
 				ra->destructor(sk);
···
 			return 0;
 		}
 	}
-	if (!new_ra)
+	if (!new_ra) {
+		spin_unlock_bh(&ip_ra_lock);
 		return -ENOBUFS;
+	}
 	new_ra->sk = sk;
 	new_ra->destructor = destructor;
 
 	RCU_INIT_POINTER(new_ra->next, ra);
 	rcu_assign_pointer(*rap, new_ra);
 	sock_hold(sk);
+	spin_unlock_bh(&ip_ra_lock);
 
 	return 0;
 }