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

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Message-ID: <70be7d61-a6fe-e703-978a-d17f544efb44@gmail.com>
Date:   Wed, 1 May 2019 08:52:37 -0600
From:   David Ahern <dsahern@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Eric Dumazet <eric.dumazet@...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 5/1/19 7:38 AM, Tetsuo Handa wrote:
> On 2019/04/30 3:43, David Ahern wrote:
>>> 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>
>>>
>>> fg
>>> <ctrl-c on perf-record>
>>>
>>> perf script
>>>
>>
>> with the header file this time.
>>
> 
> What is the intent of your patch? I can see that many notifiers are called. But
> how does this help identify which event is responsible for dropping the refcount?
> 

In a previous response you stated: "Since I'm not a netdev person, I
appreciate if you can explain that shutdown sequence using a flow chart."

The notifier sequence tells you the order of cleanup handlers and what
happens when a namespace is destroyed.

The dev_hold / dev_put tracepoint helps find the refcnt leak but
requires some time analyzing the output to match up hold / put stack traces.