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[PATCH] optimize check in port-allocation code

To: netdev@xxxxxxxxxxx
Subject: [PATCH] optimize check in port-allocation code
From: folkert@xxxxxxxxxxxxxx
Date: Wed, 20 Apr 2005 20:44:21 +0200
Organization: www.unixexpert.nl
Read-receipt-to: <folkert@vanheusden.com>
Reply-by: Thu Apr 21 15:59:59 CEST 2005
Sender: netdev-bounce@xxxxxxxxxxx
User-agent: Mutt/1.5.9i
Hi,

In the 2.6.11 code, I found this:
static int tcp_v6_get_port(struct sock *sk, unsigned short snum)
also in:
static int tcp_v4_get_port(struct sock *sk, unsigned short snum)
{
...
        if (snum == 0) {
                int low = sysctl_local_port_range[0];
                int high = sysctl_local_port_range[1];

                spin_lock(&tcp_portalloc_lock);
                rover = tcp_port_rover;
                do {    rover++;
                        if ((rover < low) || (rover > high))
                                rover = low;

That 'rover < low' check is redundant?
ints are bigger then the maximum portnumber (65535) so when
rover++ gets too high, the check for 'rover > high' will truncate
it to low (in the next line) waaay before the int itself wraps.
It is needed because tcp_port_rover might be < low before the
function starts, in that case the check for <low can be taken out
of the loop.

Patch:

diff -uNr net/ipv4/tcp_ipv4.c.org net/ipv4/tcp_ipv4.c
--- net/ipv4/tcp_ipv4.c.org     2005-03-04 22:39:37.340950747 +0100
+++ net/ipv4/tcp_ipv4.c 2005-03-04 22:40:35.570059217 +0100
@@ -222,10 +222,13 @@
                int rover;

                spin_lock(&tcp_portalloc_lock);
-               rover = tcp_port_rover;
+               if (tcp_port_rover < low)
+                       rover = low;
+               else
+                       rover = tcp_port_rover;
                do {
                        rover++;
-                       if (rover < low || rover > high)
+                       if (rover > high)
                                rover = low;
                        head = &tcp_bhash[tcp_bhashfn(rover)];
                        spin_lock(&head->lock);

diff -uNr net/ipv6/tcp_ipv6.c.org net/ipv6/tcp_ipv6.c
--- net/ipv6/tcp_ipv6.c.org     2005-03-04 22:41:44.043007791 +0100
+++ net/ipv6/tcp_ipv6.c 2005-03-04 22:42:17.604728073 +0100
@@ -139,9 +139,12 @@
                int rover;

                spin_lock(&tcp_portalloc_lock);
-               rover = tcp_port_rover;
+               if (tcp_port_rover < low)
+                       rover = low;
+               else
+                       rover = tcp_port_rover;
                do {    rover++;
-                       if ((rover < low) || (rover > high))
+                       if (rover > high)
                                rover = low;
                        head = &tcp_bhash[tcp_bhashfn(rover)];
                        spin_lock(&head->lock);


Signed-off-by: Folkert van Heusden <folkert@xxxxxxxxxxxxxx>
(please cc me, i'm not (yet) on the list)


Folkert van Heusden

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