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

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Date:   Sun, 12 Aug 2018 11:45:10 +0300
From:   Tariq Toukan <tariqt@...lanox.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        "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 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 ?