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

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Message-ID: <6ed9d782-13dd-d5b5-6b65-e583356c06d5@mellanox.com>
Date:   Wed, 18 Jul 2018 17:13:54 +0300
From:   Tariq Toukan <tariqt@...lanox.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Tariq Toukan <tariqt@...lanox.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 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?