[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>

1 files changed, 7 insertions, 2 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 5d91213d34c..67c670886c1 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/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