udp: udp_rmem_release() should touch sk_rmem_alloc later

In flood situations, keeping sk_rmem_alloc at a high value
prevents producers from touching the socket.

It makes sense to lower sk_rmem_alloc only at the end
of udp_rmem_release() after the thread draining receive
queue in udp_recvmsg() finished the writes to sk_forward_alloc.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -1191,13 +1191,14 @@ static void udp_rmem_release(struct sock *sk, int size, int partial)
 	}
 	up->forward_deficit = 0;
 
-	atomic_sub(size, &sk->sk_rmem_alloc);
 	sk->sk_forward_alloc += size;
 	amt = (sk->sk_forward_alloc - partial) & ~(SK_MEM_QUANTUM - 1);
 	sk->sk_forward_alloc -= amt;
 
 	if (amt)
 		__sk_mem_reduce_allocated(sk, amt >> SK_MEM_QUANTUM_SHIFT);
+
+	atomic_sub(size, &sk->sk_rmem_alloc);
 }
 
 /* Note: called with sk_receive_queue.lock held.