[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>

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 5d91213..67c6708 100644 (file)
--- 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

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index f6e288d..ef29cfd 100644 (file)
--- a/net/ipv6/tcp_ipv6.c
+++ b/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. */