netdev - Re:Re: Re: Re:Re: Re: [PATCH net] ppp: Fix a scheduling-while-atomic bug in del

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Message-ID: <858bbc5.bb7a.15dc9bf024c.Coremail.gfree.wind@vip.163.com>
Date:   Thu, 10 Aug 2017 09:25:24 +0800 (CST)
From:   "Gao Feng" <gfree.wind@....163.com>
To:     "Cong Wang" <xiyou.wangcong@...il.com>
Cc:     xeb@...l.ru, "David Miller" <davem@...emloft.net>,
        "Linux Kernel Network Developers" <netdev@...r.kernel.org>
Subject: Re:Re: Re: Re:Re: Re: [PATCH net] ppp: Fix a
 scheduling-while-atomic bug in del_chan

At 2017-08-10 02:18:44, "Cong Wang" <xiyou.wangcong@...il.com> wrote:
>On Tue, Aug 8, 2017 at 10:13 PM, Gao Feng <gfree.wind@....163.com> wrote:
>> Maybe I didn't show my explanation clearly.
>> I think it won't happen as I mentioned in the last email.
>> Because the pptp_release invokes the synchronize_rcu to make sure it, and actually there is no one which would invoke del_chan except pptp_release.
>> It is guaranteed by that the pptp_release doesn't put the sock refcnt until complete all cleanup include marking sk_state as PPPOX_DEAD.
>>
>> In other words, even though the pptp_release is not the last user of this sock, the other one wouldn't invoke del_chan in pptp_sock_destruct.
>> Because the condition "!(sk->sk_state & PPPOX_DEAD)" must be false.
>
>Only if sock->sk is always non-NULL for pptp_release(), which
>is what I am not sure. If you look at other ->release(), similar checks
>are there too, so not just for pptp.

Yes. It seems only if the release() is invoked twice, the sock->sk would be NULL.
But I don't find there is any case which could cause it.

>
>>
>> As summary, the del_chan and pppox_unbind_sock in pptp_sock_destruct are unnecessary.
>> And it even brings confusing.
>
>Sorry, I can't draw any conclusion for this.

Thank you all the same, and I have learn a lot from you :)
Wish someone which is familiar with these codes could give more details and explanations.

Best Regards
Feng