netdev - Re: net: deadlock between ip_expire/sch_direct

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Message-ID: <CACT4Y+bEk8TFZDcuH9S6AJ0A8uJs7ptS3ELHe-DcxkEE3Oanhg@mail.gmail.com>
Date:   Tue, 14 Mar 2017 15:58:31 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Eric Dumazet <edumazet@...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 3:56 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> 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...


Ah, I see. Thanks.