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.
