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

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Date:   Fri, 22 May 2020 08:57:56 -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>,
        Eric Dumazet <eric.dumazet@...il.com>,
        syzbot <syzkaller@...glegroups.com>,
        tipc-discussion@...ts.sourceforge.net
Subject: Re: [PATCH net] tipc: block BH before using dst_cache



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.

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.