netdev - Re: Problem: BUG_ON hit in ppp_pernet() when re-connect after changing shared key on LAC

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Date:	Wed, 6 Jul 2016 00:05:05 +0000
From:	Matt Bennett <Matt.Bennett@...iedtelesis.co.nz>
To:	Cong Wang <xiyou.wangcong@...il.com>
CC:	"linux-ppp@...r.kernel.org" <linux-ppp@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"g.nault@...halink.fr" <g.nault@...halink.fr>,
	"g.nault@...halink.fr" <g.nault@...halink.fr>
Subject: Re: Problem: BUG_ON hit in ppp_pernet() when re-connect after
 changing shared key on LAC

On 07/06/2016 08:37 AM, Cong Wang wrote:
> On Tue, Jul 5, 2016 at 10:59 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>> On Mon, Jul 4, 2016 at 7:50 PM, Matt Bennett
>> <Matt.Bennett@...iedtelesis.co.nz> wrote:
>>> Using printk I have confirmed that ppp_pernet() is called from
>>> ppp_connect_channel() when the BUG occurs (i.e. pch->chan_net is NULL).
>>>
>>> This behavior appears to have been introduced in commit 1f461dc ("ppp:
>>> take reference on channels netns").
>>
>> We have some race condition here, where a parallel ppp_unregister_channel()
>> could happen while we are in ppp_connect_channel().
>>
>> We need some synchronization for them. I am not sure what is the right lock
>> here since ppp locking looks crazy.
>
> Matt, could you try if the attached patch helps?
>
> Thanks!
>
I have given that patch a good amount of testing and the BUG_ON() no 
longer is hit. Whether that is the best fix or not I am unsure?

Either way, the following comment in ppp_unregister_channel() seems 
incorrect to me and should probably be deleted unless it is fixed?

/*
  * This ensures that we have returned from any calls into the
  * the channel's start_xmit or ioctl routine before we proceed.
  */

It appears mutex_lock(&ppp_mutex) what locks ppp_ioctl. ppp_xmit uses 
ppp_xmit_lock(ppp) in ppp_xmit_process.