netdev - Re: [PATCH net] tipc: block BH before using dst

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Date:   Fri, 22 May 2020 17:37:23 -0400
From:   Jon Maloy <jmaloy@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Xin Long <lucien.xin@...il.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        syzbot <syzkaller@...glegroups.com>,
        tipc-discussion@...ts.sourceforge.net
Subject: Re: [PATCH net] tipc: block BH before using dst_cache



On 5/22/20 4:10 PM, Eric Dumazet wrote:
>
> On 5/22/20 12:47 PM, Jon Maloy wrote:
>>
>> On 5/22/20 11:57 AM, Eric Dumazet wrote:
>>> On 5/22/20 8:01 AM, Jon Maloy wrote:
>>>> On 5/22/20 2:18 AM, Xin Long wrote:
>>>>> On Fri, May 22, 2020 at 1:55 PM Eric Dumazet <edumazet@...gle.com> wrote:
>>>>>> Resend to the list in non HTML form
>>>>>>
>>>>>>
>>>>>> On Thu, May 21, 2020 at 10:53 PM Eric Dumazet <edumazet@...gle.com> wrote:
>>>>>>> On Thu, May 21, 2020 at 10:50 PM Xin Long <lucien.xin@...il.com> wrote:
>>>>>>>> On Fri, May 22, 2020 at 2:30 AM Eric Dumazet <edumazet@...gle.com> wrote:
>>>>>>>>> dst_cache_get() documents it must be used with BH disabled.
>>>>>>>> Interesting, I thought under rcu_read_lock() is enough, which calls
>>>>>>>> preempt_disable().
>>>>>>> rcu_read_lock() does not disable BH, never.
>>>>>>>
>>>>>>> And rcu_read_lock() does not necessarily disable preemption.
>>>>> Then I need to think again if it's really worth using dst_cache here.
>>>>>
>>>>> Also add tipc-discussion and Jon to CC list.
>>>> The suggested solution will affect all bearers, not only UDP, so it is not a good.
>>>> Is there anything preventing us from disabling preemtion inside the scope of the rcu lock?
>>>>
>>>> ///jon
>>>>
>>> BH is disabled any way few nano seconds later, disabling it a bit earlier wont make any difference.
>> The point is that if we only disable inside tipc_udp_xmit() (the function pointer call) the change will only affect the UDP bearer, where dst_cache is used.
>> The corresponding calls for the Ethernet and Infiniband bearers don't use dst_cache, and don't need this disabling. So it does makes a difference.
>>
> I honestly do not understand your concern, this makes no sense to me.
>
> I have disabled BH _right_ before the dst_cache_get(cache) call, so has no effect if the dst_cache is not used, this should be obvious.
Forget my comment. I thought we were discussing to Tetsuo Handa's 
original patch, and missed that you had posted your own.
I have no problems with this one.

///jon

>
> If some other paths do not use dst)cache, how can my patch have any effect on them ?
>
> What alternative are you suggesting ?
>