Re: [PATCH v2 net] net: sch_generic: aviod concurrent reset and enqueue op for lockless qdisc

[Yunsheng Lin]

On 2020/10/29 4:04, Vishwanath Pai wrote:
> On 10/28/20 1:47 PM, Cong Wang wrote:
>> On Wed, Oct 28, 2020 at 8:37 AM Pai, Vishwanath <vpai@xxxxxxxxxx> wrote:
>>> Hi,
>>>
>>> We noticed some problems when testing the latest 5.4 LTS kernel and traced it
>>> back to this commit using git bisect. When running our tests the machine stops
>>> responding to all traffic and the only way to recover is a reboot. I do not see
>>> a stack trace on the console.
>>
>> Do you mean the machine is still running fine just the network is down?
>>
>> If so, can you dump your tc config with stats when the problem is happening?
>> (You can use `tc -s -d qd show ...`.)
>>
>>>
>>> This can be reproduced using the packetdrill test below, it should be run a
>>> few times or in a loop. You should hit this issue within a few tries but
>>> sometimes might take up to 15-20 tries.
>> ...
>>> I can reproduce the issue easily on v5.4.68, and after reverting this commit it
>>> does not happen anymore.
>>
>> This is odd. The patch in this thread touches netdev reset path, if packetdrill
>> is the only thing you use to trigger the bug (that is netdev is always active),
>> I can not connect them.
>>
>> Thanks.
> 
> Hi Cong,
> 
>> Do you mean the machine is still running fine just the network is down?
> 
> I was able to access the machine via serial console, it looks like it is
> up and running, just that networking is down.
> 
>> If so, can you dump your tc config with stats when the problem is happening?
>> (You can use `tc -s -d qd show ...`.)
> 
> If I try running tc when the machine is in this state the command never
> returns. It doesn't print anything but doesn't exit either.
> 
>> This is odd. The patch in this thread touches netdev reset path, if packetdrill
>> is the only thing you use to trigger the bug (that is netdev is always active),
>> I can not connect them.
> 
> I think packetdrill creates a tun0 interface when it starts the
> test and tears it down at the end, so it might be hitting this code path
> during teardown.

Hi, Is there any preparation setup before running the above packetdrill test
case, I run the above test case in 5.9-rc4 with this patch applied without any
preparation setup, did not reproduce it.

By the way, I am newbie to packetdrill:), it would be good to provide the
detail setup to reproduce it,thanks.

> 
> P.S: My mail server is having connectivity issues with vger.kernel.org
> so messages aren't getting delivered to netdev. It'll hopefully get
> resolved soon.
> 
> Thanks,
> Vishwanath
> 
> 
> .
>