kern/183732: igb interface output byte counter double real output bytes on 8.4-RELEASE generic kernel

[Andrew Kinney]

>Number:         183732
>Category:       kern
>Synopsis:       igb interface output byte counter double real output bytes on 8.4-RELEASE generic kernel
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Thu Nov 07 01:10:00 UTC 2013
>Closed-Date:
>Last-Modified:
>Originator:     Andrew Kinney
>Release:        8.4-RELEASE
>Organization:
Advantagecom Networks, Inc.
>Environment:
FreeBSD redact.redact.com 8.4-RELEASE-p4 FreeBSD 8.4-RELEASE-p4 #0: Mon Sep  9 23:16:13 UTC 2013     root at amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC  amd64
>Description:
With a generic unmodified 8.4-RELEASE-p4 kernel, igb interface outbound byte counters are double real output bytes. Packet counts are as expected.

This existing problem report against 8.4-STABLE seems to identify the root cause, but it was largely ignored because it wasn't reported against 8.4-RELEASE:
http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/182828

Symptoms visible on a box running bridging and IPFW with two igb interfaces. Outbound bytes is usually about double inbound bytes on the opposite interface. Packet counts are about equal. Very visible when there is significant traffic graphed via snmpd and cacti.

Diagnosed with:
netstat -I igb0 -btW ; netstat -I igb2 -btW ; date
# wait a few seconds
netstat -I igb0 -btW ; netstat -I igb2 -btW ; date
# compare Ibytes delta on one interface to Obytes delta on opposite interface

Actual Results: Obytes delta on one interface should be roughly equal to Ibytes delta on the opposite interface.

Expected Results: Ibytes on one interface should be roughly equal to Obytes on the opposite interface
>How-To-Repeat:
Use FreeBSD-8.4-RELEASE with igb interfaces. Observe Obytes from "netstat -I igb0 -btW" with a known quantity of network traffic before and after the traffic. Obytes will be approximately double the expected value.
>Fix:
patch submitted against FreeBSD-8.4-STABLE on
http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/182828

>Release-Note:
>Audit-Trail:
>Unformatted: