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Date:   Fri, 22 May 2020 08:57:56 -0700
From:   Eric Dumazet <>
To:     Jon Maloy <>, Xin Long <>,
        Eric Dumazet <>
Cc:     "David S . Miller" <>,
        netdev <>,
        Eric Dumazet <>,
        syzbot <>,
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 <> wrote:
>>> Resend to the list in non HTML form
>>> On Thu, May 21, 2020 at 10:53 PM Eric Dumazet <> wrote:
>>>> On Thu, May 21, 2020 at 10:50 PM Xin Long <> wrote:
>>>>> On Fri, May 22, 2020 at 2:30 AM Eric Dumazet <> 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.

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