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Subject: Re: [PATCH v2 net] net: sch_generic: aviod concurrent reset and
 enqueue op for lockless qdisc
To:     Cong Wang <xiyou.wangcong@gmail.com>
Cc:     Yunsheng Lin <linyunsheng@huawei.com>,
        "Hunt, Joshua" <johunt@akamai.com>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Jiri Pirko <jiri@resnulli.us>,
        David Miller <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Linux Kernel Network Developers <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "linuxarm@huawei.com" <linuxarm@huawei.com>,
        John Fastabend <john.fastabend@gmail.com>,
        Eric Dumazet <eric.dumazet@gmail.com>
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From:   Vishwanath Pai <vpai@akamai.com>
Message-ID: <19f888c2-8bc1-ea56-6e19-4cb4841c4da0@akamai.com>
Date:   Wed, 28 Oct 2020 16:04:52 -0400
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On 10/28/20 1:47 PM, Cong Wang wrote:
 > On Wed, Oct 28, 2020 at 8:37 AM Pai, Vishwanath <vpai@akamai.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.

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