netdev - Re: [PATCH net] net/xdp: Fix suspicious RCU usage warning

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Date:   Mon, 13 Aug 2018 01:04:40 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Tariq Toukan <tariqt@...lanox.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Eran Ben Elisha <eranbe@...lanox.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH net] net/xdp: Fix suspicious RCU usage warning

On 08/12/2018 10:45 AM, Tariq Toukan wrote:
> 
> 
> On 20/07/2018 12:36 AM, Alexei Starovoitov wrote:
>> On Wed, Jul 18, 2018 at 05:13:54PM +0300, Tariq Toukan wrote:
>>>
>>>
>>> On 17/07/2018 10:27 PM, Daniel Borkmann wrote:
>>>> On 07/17/2018 06:47 PM, Alexei Starovoitov wrote:
>>>>> On Tue, Jul 17, 2018 at 06:10:38PM +0300, Tariq Toukan wrote:
>>>>>> Fix the warning below by calling rhashtable_lookup under
>>>>>> RCU read lock.
>>>>>>
>>>
>>> ...
>>>
>>>>>>        mutex_lock(&mem_id_lock);
>>>>>> +    rcu_read_lock();
>>>>>>        xa = rhashtable_lookup(mem_id_ht, &id, mem_id_rht_params);
>>>>>> +    rcu_read_unlock();
>>>>>>        if (!xa) {
>>>>>
>>>>> if it's an actual bug rcu_read_unlock seems to be misplaced.
>>>>> It silences the warn, but rcu section looks wrong.
>>>>
>>>> I think that whole piece in __xdp_rxq_info_unreg_mem_model() should be:
>>>>
>>>>     mutex_lock(&mem_id_lock);
>>>>     xa = rhashtable_lookup_fast(mem_id_ht, &id, mem_id_rht_params);
>>>>     if (xa && rhashtable_remove_fast(mem_id_ht, &xa->node, mem_id_rht_params) == 0)
>>>>             call_rcu(&xa->rcu, __xdp_mem_allocator_rcu_free);
>>>>     mutex_unlock(&mem_id_lock);
>>>>
>>>> Technically the RCU read side plus rhashtable_lookup() is the same, but lets
>>>> use proper api. From the doc (https://lwn.net/Articles/751374/) object removal
>>>> is wrapped around the RCU read side additionally, but in our case we're behind
>>>> mem_id_lock for insertion/removal serialization.
>>>>
>>>> Cheers,
>>>> Daniel
>>>>
>>>
>>> Just as Daniel stated, I think there's no actual bug here, but we still want
>>> to silence the RCU warning.
>>>
>>> Alexei, did you mean getting the if statement into the RCU lock critical
>>> section?
>>
>> If what Daniel proposes silences the warn, I'd rather do that.
>> Pattern like:
>>    rcu_lock;
>>    val = lookup();
>>    rcu_unlock;
>>    if (val)
>> will cause people to question the quality of the code and whether
>> authors of the code understand rcu.
>> There should be a way to silence the warn without adding
>> "wrong on the first glance" code.
> 
> I'm re-spinning this.
> Can it still go to net, or better send it to bpf-next ?

Please rebase against bpf-next and we route it to stable, thanks!