netdev - help on iproute2 hangs

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Date:   Thu, 1 Mar 2018 09:29:59 -0800
From:   William Tu <u9012063@...il.com>
To:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Cc:     chrism@...lanox.com, David Ahern <dsahern@...il.com>
Subject: help on iproute2 hangs

Hi,

We're running commands below on kernel 4.15.0:
1) ip netns add at_ns0
2) ip link add p0 type veth peer name ovs-p0
3) ip link set p0 netns at_ns0
4) ip link set dev ovs-p0 up

However, it always hangs at creating veth peer, command (2) when we
run above commands in GNU autotest. Running the same commands in a
bash script works fine.

Running bisect on iproute2, the first bad commit is
commit 72a2ff3916e59d2132a7d613d9e8f5eb372ba43e
Author: Chris Mi <chrism@...lanox.com>
Date:   Fri Jan 12 14:13:15 2018 +0900

   lib/libnetlink: Add a new function rtnl_talk_iov

   rtnl_talk can only send a single message to kernel. Add a new function
   rtnl_talk_iov that can send multiple messages to kernel.
   rtnl_talk_iov takes struct iovec * and iovlen as arguments.

GDB shows it hangs on
#0  0x00007f9c92e6b330 in recvmsg () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x000000000044eac6 in __rtnl_recvmsg ()
#2  0x000000000044eb95 in rtnl_recvmsg ()
#3  0x000000000044ed46 in __rtnl_talk_iov ()
#4  0x000000000044fbb8 in rtnl_talk ()
#5  0x0000000000422554 in iplink_modify ()

Any suggestion about how to debug this?
Thanks
William