Re: [PATCH] ipv4: fix a RCU-list lock in fib_triestat_seq_show

[Eric Dumazet <eric.dumazet@gmail.com>]

On 3/25/20 10:34 AM, Qian Cai wrote:
> 
> 
>> On Mar 25, 2020, at 12:13 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>>
>> I would prefer :
>>
>> diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c
>> index ff0c24371e3309b3068980f46d1ed743337d2a3e..4b98ffb27136d3b43f179d6b1b42fe84586acc06 100644
>> --- a/net/ipv4/fib_trie.c
>> +++ b/net/ipv4/fib_trie.c
>> @@ -2581,6 +2581,7 @@ static int fib_triestat_seq_show(struct seq_file *seq, void *v)
>>                struct hlist_head *head = &net->ipv4.fib_table_hash[h];
>>                struct fib_table *tb;
>>
>> +               rcu_read_lock();
>>                hlist_for_each_entry_rcu(tb, head, tb_hlist) {
>>                        struct trie *t = (struct trie *) tb->tb_data;
>>                        struct trie_stat stat;
>> @@ -2596,6 +2597,7 @@ static int fib_triestat_seq_show(struct seq_file *seq, void *v)
>>                        trie_show_usage(seq, t->stats);
>> #endif
>>                }
>> +               rcu_read_unlock();
>>        }
>>
>>        return 0;
> 
> I have no strong opinion either way. My initial thought was to save 255 extra lock/unlock with a single lock/unlock, but I am not sure how time-consuming for each iteration of the outer loop could be. If it could take a bit too long, it does make a lot of sense to reduce the critical section.
> 


This file could be quite big in some setups.

Alternatively you could use cond_resched_rcu()