Description of problem:
When a domU is configured to have multiple network interfaces, it may fail to create the iptables rules to allow each of the domU's interfaces to bridge the dom0's firewall.

It seems to be a resources issue with iptables and the Xen scripts for networking - multiple network interfaces means that the iptables command is run in a very quick sequence or even concurrently to add vif PHYSDEV match rules resulting in resource contention.

Version-Release number of selected component (if applicable):


How reproducible:
A domU with 2 network interfaces I observed the problem 50% of the time.
A domU with 3 network interfaces I observed the problem 100% of the time 

Steps to Reproduce:
1. Set up a domU guest with more than one network interface - for example edit the /etc/xen/domuGuest  file and add line :
vif = [ '', '', '' ]

2. Start the domU with xm create 

  
Actual results:

After starting the domU we observe the iptables firewall rules :

ACCEPT     all  --  anywhere             anywhere            PHYSDEV match --physdev-in vif1.0
ACCEPT     all  --  anywhere             anywhere            PHYSDEV match --physdev-in vif1.1

And in the /var/log/messages we see:

iptables -A FORWARD -m physdev --physdev-in BLAH BLAH -j ACCEPT failed.
If you are using iptables, this may affect networking for guest domains.

If we strace the iptables command itself we see:

5087  setsockopt(3, SOL_IP, 0x40 /* IP_??? */, "filter.., 3840) = -1 EAGAIN (Resource temporarily unavailable)
5087  write(2, "iptables: Unknown error 18446744073709551615n", 45) = 45
5087  exit_group(1)                     = ?

Expected results:
We should see the xm create command for the domU run completely, create the domain and the PHYSDEV match rules for each vif should be present. Also no errors in /var/log/messages  

Additional info:

A patch to resolve the problem is attached. I adds a simple locking mechanism around the iptables command as it is run by the xen network scripts. It ensures that the iptables commands are  run one at a time.