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

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Date:   Thu, 19 Jul 2018 14:36:29 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     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 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.