linux-kernel - Re: unregister_netdevice: waiting for DEV to become free (2)

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Message-ID: <6e57bc11-1603-0898-dfd4-0f091901b422@i-love.sakura.ne.jp>
Date:   Sun, 28 Apr 2019 13:22:49 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        David Ahern <dsahern@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Julian Anastasov <ja@....bg>, Cong Wang <xiyou.wangcong@...il.com>,
        syzbot <syzbot+30209ea299c09d8785c9@...kaller.appspotmail.com>,
        ddstreet@...e.org, dvyukov@...gle.com,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: unregister_netdevice: waiting for DEV to become free (2)

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.