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!
