[UNIX]: The unix_nr_socks limit can be exceeded

The unix_nr_socks value is limited with the 2 * get_max_files() value,
as seen from the unix_create1(). However, the check and the actual
increment are separated with the GFP_KERNEL allocation, so this limit
can be exceeded under a memory pressure - task may go to sleep freeing
the pages and some other task will be allowed to allocate a new sock
and so on and so forth.

So make the increment before the check (similar thing is done in the
sock_kmalloc) and go to kmalloc after this.
Signed-off-by: NPavel Emelyanov <xemul@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/unix/af_unix.c

@@ -599,15 +599,14 @@ static struct sock * unix_create1(struct net *net, struct socket *sock)
 	struct sock *sk = NULL;
 	struct unix_sock *u;
 
-	if (atomic_read(&unix_nr_socks) >= 2*get_max_files())
+	atomic_inc(&unix_nr_socks);
+	if (atomic_read(&unix_nr_socks) > 2 * get_max_files())
 		goto out;
 
 	sk = sk_alloc(net, PF_UNIX, GFP_KERNEL, &unix_proto);
 	if (!sk)
 		goto out;
 
-	atomic_inc(&unix_nr_socks);
-
 	sock_init_data(sock,sk);
 	lockdep_set_class(&sk->sk_receive_queue.lock,
 				&af_unix_sk_receive_queue_lock_key);
@@ -625,6 +624,8 @@ static struct sock * unix_create1(struct net *net, struct socket *sock)
 	init_waitqueue_head(&u->peer_wait);
 	unix_insert_socket(unix_sockets_unbound, sk);
 out:
+	if (sk == NULL)
+		atomic_dec(&unix_nr_socks);
 	return sk;
 }