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

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Date:   Fri, 22 May 2020 15:47:16 -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 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.
///jon

>
> Also, if you intend to make dst_cache BH reentrant, you will have to make that for net-next, not net tree.
>
> Please carefully read include/net/dst_cache.h
>
> It is very clear about BH requirements.
>
>