netdev - Re: [PATCH 1/1] ppp: Fix one deadlock issue of PPP when send frame

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Date:   Fri, 19 Aug 2016 06:53:49 +0800
From:   Feng Gao <gfree.wind@...il.com>
To:     Philp Prindeville <philipp@...fish-solutions.com>
Cc:     Gao Feng <fgao@...vckh6395k16k5.yundunddos.com>, paulus@...ba.org,
        linux-ppp@...r.kernel.org,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/1] ppp: Fix one deadlock issue of PPP when send frame

Hi Philp,

Yes. I am agree with you.
Just drop is better to support recursive lock.

I will send a new patch later.

Regards
Feng


On Fri, Aug 19, 2016 at 12:48 AM, Philp Prindeville
<philipp@...fish-solutions.com> wrote:
>
>
> On 08/18/2016 09:05 AM, Feng Gao wrote:
>>
>> On Thu, Aug 18, 2016 at 10:11 PM, Philp Prindeville
>> <philipp@...fish-solutions.com>  wrote:
>>>
>>> >Feng,
>>> >
>>> >If the CPU can already be holding the lock, that implies re-entrancy.
>>> >What's to stop the first flow of code which acquired the lock from
>>> > releasing
>>> >it again before the 2nd flow is done?  Is the 2nd flow running at a
>>> > higher
>>> >priority or with interrupts disabled?
>>
>> There is no preemption happened. It is caused by wrong route policy by
>> l2tp.
>> For example, the cpu0 get the spinlock of channel1, then the channel1
>> is selected again after route. As a result, cpu0 tries to get the same
>> spinlock again.
>>
>> The call flow is like this.
>> ppp_write->ppp_channel_push->start_xmit->select inappropriate route
>> .... -> dev_hard_start_xmit->ppp_start_xmit->ppp_xmit_process->
>> ppp_push. Now ppp_push tries to get the same spinlock which is held
>> in ppp_channel_push.
>>
>> Regards
>> Feng
>>
>
> If we're detecting (through the fact that the lock has already been
> acquired) that the wrong route is being applied, why don't we just punt the
> packet instead?
>
> -Philip
>