[TCP]: Document non-trivial locking path in tcp_v{4,6}_get_port().

This trips up a lot of folks reading this code.
Put an unlikely() around the port-exhaustion test
for good measure.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -242,9 +242,14 @@ static int tcp_v4_get_port(struct sock *sk, unsigned short snum)
 		tcp_port_rover = rover;
 		spin_unlock(&tcp_portalloc_lock);
 
-		/* Exhausted local port range during search? */
+		/* Exhausted local port range during search?  It is not
+		 * possible for us to be holding one of the bind hash
+		 * locks if this test triggers, because if 'remaining'
+		 * drops to zero, we broke out of the do/while loop at
+		 * the top level, not from the 'break;' statement.
+		 */
 		ret = 1;
-		if (remaining <= 0)
+		if (unlikely(remaining <= 0))
 			goto fail;
 
 		/* OK, here is the one we will use.  HEAD is

net/ipv6/tcp_ipv6.c

@@ -158,9 +158,14 @@ static int tcp_v6_get_port(struct sock *sk, unsigned short snum)
 		tcp_port_rover = rover;
 		spin_unlock(&tcp_portalloc_lock);
 
-		/* Exhausted local port range during search? */
+		/* Exhausted local port range during search?  It is not
+		 * possible for us to be holding one of the bind hash
+		 * locks if this test triggers, because if 'remaining'
+		 * drops to zero, we broke out of the do/while loop at
+		 * the top level, not from the 'break;' statement.
+		 */
 		ret = 1;
-		if (remaining <= 0)
+		if (unlikely(remaining <= 0))
 			goto fail;
 
 		/* OK, here is the one we will use. */