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Subject: Re: INFO: rcu detected stall in wg_packet_tx_worker
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     syzbot <syzbot+0251e883fe39e7a0cb0a@syzkaller.appspotmail.com>,
        David Miller <davem@davemloft.net>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        jhs@mojatatu.com,
        =?UTF-8?B?SmnFmcOtIFDDrXJrbw==?= <jiri@resnulli.us>,
        Krzysztof Kozlowski <krzk@kernel.org>, kuba@kernel.org,
        kvalo@codeaurora.org, leon@kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        linux-kselftest@vger.kernel.org, Netdev <netdev@vger.kernel.org>,
        Shuah Khan <shuah@kernel.org>, syzkaller-bugs@googlegroups.com,
        Thomas Gleixner <tglx@linutronix.de>, vivien.didelot@gmail.com,
        Cong Wang <xiyou.wangcong@gmail.com>
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 <f2eb18ea-b32a-4b64-0417-9b5b2df98e33@gmail.com>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Message-ID: <29bd64f4-5fe0-605e-59cc-1afa199b1141@gmail.com>
Date:   Sun, 26 Apr 2020 13:38:37 -0700
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On 4/26/20 1:26 PM, Eric Dumazet wrote:
> 
> 
> On 4/26/20 12:42 PM, Jason A. Donenfeld wrote:
>> On Sun, Apr 26, 2020 at 1:40 PM Eric Dumazet <eric.dumazet@gmail.com> wrote:
>>>
>>>
>>>
>>> On 4/26/20 10:57 AM, syzbot wrote:
>>>> syzbot has bisected this bug to:
>>>>
>>>> commit e7096c131e5161fa3b8e52a650d7719d2857adfd
>>>> Author: Jason A. Donenfeld <Jason@zx2c4.com>
>>>> Date:   Sun Dec 8 23:27:34 2019 +0000
>>>>
>>>>     net: WireGuard secure network tunnel
>>>>
>>>> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15258fcfe00000
>>>> start commit:   b2768df2 Merge branch 'for-linus' of git://git.kernel.org/..
>>>> git tree:       upstream
>>>> final crash:    https://syzkaller.appspot.com/x/report.txt?x=17258fcfe00000
>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=13258fcfe00000
>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=b7a70e992f2f9b68
>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=0251e883fe39e7a0cb0a
>>>> userspace arch: i386
>>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15f5f47fe00000
>>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11e8efb4100000
>>>>
>>>> Reported-by: syzbot+0251e883fe39e7a0cb0a@syzkaller.appspotmail.com
>>>> Fixes: e7096c131e51 ("net: WireGuard secure network tunnel")
>>>>
>>>> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
>>>>
>>>
>>> I have not looked at the repro closely, but WireGuard has some workers
>>> that might loop forever, cond_resched() might help a bit.
>>
>> I'm working on this right now. Having a bit difficult of a time
>> getting it to reproduce locally...
>>
>> The reports show the stall happening always at:
>>
>> static struct sk_buff *
>> sfq_dequeue(struct Qdisc *sch)
>> {
>>        struct sfq_sched_data *q = qdisc_priv(sch);
>>        struct sk_buff *skb;
>>        sfq_index a, next_a;
>>        struct sfq_slot *slot;
>>
>>        /* No active slots */
>>        if (q->tail == NULL)
>>                return NULL;
>>
>> next_slot:
>>        a = q->tail->next;
>>        slot = &q->slots[a];
>>
>> Which is kind of interesting, because it's not like that should block
>> or anything, unless there's some kasan faulting happening.
>>
> 
> I am not really sure WireGuard is involved, the repro does not rely on it anyway.
> 

Yes, do not spend too much time on this.

syzbot found its way into crazy qdisc settings these last days.

( I sent a patch yesterday for choke qdisc, it seems similar checks are needed in sfq )