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SubjectRe: INFO: rcu detected stall in llcp_sock_sendmsg
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On 07/09/2018 01:50 PM, Dmitry Vyukov wrote:
> On Mon, Jul 9, 2018 at 10:34 PM, syzbot
> <syzbot+e9f364d3b15ce41d8451@syzkaller.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@syzkaller.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.


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