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Date:   Mon, 29 Apr 2019 12:34:54 -0600
From:   David Ahern <>
To:     Tetsuo Handa <>,
        Eric Dumazet <>,
        "David S. Miller" <>
Cc:     Julian Anastasov <>, Cong Wang <>,
        syzbot <>,,,,,
Subject: Re: unregister_netdevice: waiting for DEV to become free (2)

On 4/27/19 10:22 PM, Tetsuo Handa wrote:
> On 2019/04/28 8:52, Eric Dumazet wrote:
>> On 4/27/19 3:33 PM, Tetsuo Handa wrote:
>>> I'm waiting for davem why it is safe to move the dst entry from
>>> "a device to unregister" to "a loopback device in that namespace".
>>> I'm waiting for an explanation how the dst entry which was moved to
>>> "a loopback device in that namespace" is released (i.e. what the
>>> expected shutdown sequence is).
>> The most probable explanation is that we make sure the loopback device
>> is the last one to be dismantled at netns deletion,
>> and this would obviously happen after all dst have been released.
> rt_flush_dev() becomes a no-op if "dev" == "a loopback device in that
> namespace". And according to debug printk(), rt_flush_dev() is called
> on "a loopback device in that namespace" itself.
> If "a loopback device in that namespace" is the last "one" (== "a network
> device in that namespace" ?), which shutdown sequence should have called
> dev_put("a loopback device in that namespace") before unregistration of
> "a loopback device in that namespace" starts?
> Since I'm not a netdev person, I appreciate if you can explain
> that shutdown sequence using a flow chart.

The attached patch adds a tracepoint to notifier_call_chain. If you have
KALLSYMS enabled it will show the order of the function handlers:

perf record -e notifier:* -a -g &

ip netns del <NAME>
<wait a few seconds>

<ctrl-c on perf-record>

perf script

View attachment "0001-notifier-add-tracepoint-to-notifier_call_chain.patch" of type "text/plain" (908 bytes)

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