netdev - Re: neighbour netlink notifications delivered in wrong order

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Message-ID: <78825e0b-d157-5b26-4263-8fd367d2fb2c@nvidia.com>
Date:   Tue, 7 Jun 2022 20:49:40 -0700
From:   Andy Roulin <aroulin@...dia.com>
To:     Francesco Ruggeri <fruggeri@...sta.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Cc:     netdev@...r.kernel.org
Subject: Re: neighbour netlink notifications delivered in wrong order

On 6/7/22 1:03 PM, Francesco Ruggeri wrote:
> On Tue, Jun 7, 2022 at 10:32 AM Stephen Hemminger
> <stephen@...workplumber.org> wrote:
>>
>> On Tue, 7 Jun 2022 09:29:45 -0700
>> Francesco Ruggeri <fruggeri@...sta.com> wrote:
>>
>>> On Mon, Jun 6, 2022 at 8:19 PM Stephen Hemminger
>>> <stephen@...workplumber.org> wrote:
>>>>
>>>> On Mon, 6 Jun 2022 19:07:04 -0700
>>>> Andy Roulin <aroulin@...dia.com> wrote:
>>>>
>>>>> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
>>>>> index 54625287ee5b..a91dfcbfc01c 100644
>>>>> --- a/net/core/neighbour.c
>>>>> +++ b/net/core/neighbour.c
>>>>> @@ -2531,23 +2531,19 @@ static int neigh_fill_info(struct sk_buff *skb,
>>>>> struct neighbour *neigh,
>>>>>        if (nla_put(skb, NDA_DST, neigh->tbl->key_len, neigh->primary_key))
>>>>>                goto nla_put_failure;
>>>>>
>>>>> -     read_lock_bh(&neigh->lock);
>>>>>        ndm->ndm_state   = neigh->nud_state;
>>>>
>>>> Accessing neighbor state outside of lock is not safe.
>>>>
>>>> But you should be able to use RCU here??
>>>
>>> I think the patch removes the lock from neigh_fill_info but it then uses it
>>> to protect all calls to neigh_fill_info, so the access should still be safe.
>>> In case of __neigh_notify the lock also extends to protect rtnl_notify,
>>> guaranteeing that the state cannot be changed while the notification
>>> is in progress (I assume all state changes are protected by the same lock).
>>> Andy, is that the idea?

Yes correct.

>>
>> Neigh info is already protected by RCU, is per neighbour reader/writer lock
>> still needed at all?
> 
> The goal of the patch seems to be to make changing a neighbour's state and
> delivering the corresponding notification atomic, in order to prevent
> reordering of notifications. It uses the existing lock to do so.
> Can reordering be prevented if the lock is replaced with rcu?

Yes that's the goal of the patch. I'd have to look in more details if 
there's a better solution with RCU.