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

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Date:   Fri, 22 May 2020 13:10:03 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jon Maloy <jmaloy@...hat.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 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.

If some other paths do not use dst)cache, how can my patch have any effect on them ?

What alternative are you suggesting ?