netdev - Re: INFO: rcu detected stall in llcp_sock

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Date:   Mon, 9 Jul 2018 14:05:08 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+e9f364d3b15ce41d8451@...kaller.appspotmail.com>,
        Samuel Ortiz <sameo@...ux.intel.com>,
        David Miller <davem@...emloft.net>,
        linux-wireless@...r.kernel.org, netdev <netdev@...r.kernel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: INFO: rcu detected stall in llcp_sock_sendmsg



On 07/09/2018 01:50 PM, Dmitry Vyukov wrote:
> On Mon, Jul 9, 2018 at 10:34 PM, syzbot
> <syzbot+e9f364d3b15ce41d8451@...kaller.appspotmail.com> wrote:
>> Hello,
>>
>> syzbot found the following crash on:
>>
>> HEAD commit:    1e4b044d2251 Linux 4.18-rc4
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=1414c2c2400000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=25856fac4e580aa7
>> dashboard link: https://syzkaller.appspot.com/bug?extid=e9f364d3b15ce41d8451
>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>>
>> Unfortunately, I don't have any reproducer for this crash yet.
>>
>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>> Reported-by: syzbot+e9f364d3b15ce41d8451@...kaller.appspotmail.com
> 
> Looks like the problem is actually in nfc, so +nfc maintainers.

Note this issue was discussed before, maybe we should patch NFC without waiting for nfc maintainer.

----------------------------------------------------

On 06/25/2018 10:12 PM, Sergey Senozhatsky wrote:
> On (06/26/18 07:07), Dmitry Vyukov wrote:
> [..]
>>>  #include <net/nfc/nfc.h>
>>> @@ -755,7 +756,8 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
>>>                 pdu = nfc_alloc_send_skb(sock->dev, &sock->sk, MSG_DONTWAIT,
>>>                                          frag_len + LLCP_HEADER_SIZE, &err);
>>>                 if (pdu == NULL) {
>>> -                       pr_err("Could not allocate PDU\n");
>>> +                       pr_err_ratelimited("Could not allocate PDU\n");
>>> +                       cond_resched();
>>>                         continue;
>>>                 }
>>
>>
>> But this thread is still in an infinite (unkillable?) loop? If yes, we
>> are waiting for the next syzbot report 
>
> The loop is still infinite, correct, but we have a preemption point now.
> Sure, net people can come with a much better solution, I'll be happy to
> scratch my patch.
>

This can not be the right solution, think about current thread being real time,
cond_resched() might be a nop.

We should probably not loop at all, or not use MSG_DONTWAIT.

(And remove this useless "Could not allocate PDU" message)

NFC maintainers should really take a look at this.