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.