Date:   Thu, 29 Oct 2020 00:50:36 -0400
From:   Vishwanath Pai <vpai@...mai.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>,
        Cong Wang <xiyou.wangcong@...il.com>
Cc:     "Hunt, Joshua" <johunt@...mai.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linuxarm@...wei.com" <linuxarm@...wei.com>,
        John Fastabend <john.fastabend@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v2 net] net: sch_generic: aviod concurrent reset and
 enqueue op for lockless qdisc

On 10/28/20 10:37 PM, Yunsheng Lin wrote:
 > 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@...mai.com> 
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
 >>
 >>
 >> .
 >>

I can't reproduce it on v5.9-rc4 either, it is probably an issue only on
5.4 then (and maybe older LTS versions). Can you give it a try on
5.4.68?

For running packetdrill, download the latest version from their github
repo, then run it in a loop without any special arguments. This is what
I do to reproduce it:

while true; do ./packetdrill <test-file>; done

I don't think any other setup is necessary.

-Vishwanath

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