netdev - Re: net: deadlock between ip_expire/sch_direct

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Date:   Tue, 14 Mar 2017 07:56:56 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        James Morris <jmorris@...ei.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Patrick McHardy <kaber@...sh.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: net: deadlock between ip_expire/sch_direct_xmit

On Tue, Mar 14, 2017 at 7:46 AM, Dmitry Vyukov <dvyukov@...gle.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...