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From: Eric Dumazet <edumazet@google.com>
Date: Tue, 14 Mar 2017 07:56:56 -0700
Message-ID: <CANn89iLO1neA3-4ipYr==n_3iDXDXgY0MCkPkp=cEf8n4w6i=g@mail.gmail.com>
Subject: Re: net: deadlock between ip_expire/sch_direct_xmit
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
        David Miller <davem@davemloft.net>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        James Morris <jmorris@namei.org>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Patrick McHardy <kaber@trash.net>, netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        syzkaller <syzkaller@googlegroups.com>
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On Tue, Mar 14, 2017 at 7:46 AM, Dmitry Vyukov <dvyukov@google.com> wrote:

> I am confused. Lockdep has observed both of these stacks:
>
>        CPU0                    CPU1
>        ----                    ----
>   lock(&(&q->lock)->rlock);
>                                lock(_xmit_ETHER#2);
>                                lock(&(&q->lock)->rlock);
>   lock(_xmit_ETHER#2);
>
>
> So it somehow happened. Or what do you mean?
>

Lockdep said " possible circular locking dependency detected " .
It is not an actual deadlock, but lockdep machinery firing.

For a dead lock to happen, this would require that he ICMP message
sent by ip_expire() is itself fragmented and reassembled.
This cannot be, because ICMP messages are not candidates for
fragmentation, but lockdep can not know that of course...