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Subject: Re: unregister_netdevice: waiting for DEV to become free (2)
To:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        Julian Anastasov <ja@ssi.bg>,
        Cong Wang <xiyou.wangcong@gmail.com>
Cc:     syzbot <syzbot+30209ea299c09d8785c9@syzkaller.appspotmail.com>,
        ddstreet@ieee.org, dvyukov@google.com,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        syzkaller-bugs@googlegroups.com
References: <0000000000007d22100573d66078@google.com>
 <alpine.LFD.2.20.1808201527230.2758@ja.home.ssi.bg>
 <4684eef5-ea50-2965-86a0-492b8b1e4f52@I-love.SAKURA.ne.jp>
From:   David Ahern <dsahern@gmail.com>
Message-ID: <dc7c06ae-204d-c287-04a4-d48875476d09@gmail.com>
Date:   Mon, 15 Apr 2019 09:35:01 -0600
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On 4/15/19 7:36 AM, Tetsuo Handa wrote:
> I traced using debug printk() patch shown below.
> 

I find tracepoints (see attached patch) and perf are easier to use to
debug device refcnt problems.

For example, limit the stack you have to deal with via sysctl -w
kernel.perf_event_max_stack=16, and add a filter (e.g., --filter 'name
== "lo"') to limit collection to a specific device.

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