netdev - Re: [PATCH net-next 11/16] ipv6: replace dst_hold() with dst_hold

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Message-ID: <CAEA6p_BdnBB71eQJ7LGWDiZpkymQKJ_TcFJ78xZ8-vB94QHXyA@mail.gmail.com>
Date:   Fri, 6 Oct 2017 19:06:01 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     吉藤英明 <hideaki.yoshifuji@...aclelinux.com>
Cc:     David Miller <davem@...emloft.net>,
        network dev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>
Subject: Re: [PATCH net-next 11/16] ipv6: replace dst_hold() with
 dst_hold_safe() in routing code

On Fri, Oct 6, 2017 at 4:57 PM, 吉藤英明 <hideaki.yoshifuji@...aclelinux.com> wrote:
> Hi,
>
> 2017-10-07 4:06 GMT+09:00 Wei Wang <weiwan@...gle.com>:
>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>> index 941c062389d2..aeb349aea429 100644
>> --- a/net/ipv6/route.c
>> +++ b/net/ipv6/route.c
> :
>> @@ -1625,12 +1643,17 @@ struct rt6_info *ip6_pol_route(struct net *net, struct fib6_table *table,
>>         if (rt_cache)
>>                 rt = rt_cache;
>>
>> -       if (rt == net->ipv6.ip6_null_entry || (rt->rt6i_flags & RTF_CACHE)) {
>> -               dst_use(&rt->dst, jiffies);
>> +       if (rt == net->ipv6.ip6_null_entry) {
>> +               read_unlock_bh(&table->tb6_lock);
>> +               dst_hold(&rt->dst);
>> +               trace_fib6_table_lookup(net, rt, table->tb6_id, fl6);
>> +               return rt;
>> +       } else if (rt->rt6i_flags & RTF_CACHE) {
>> +               if (ip6_hold_safe(net, &rt, true)) {
>> +                       dst_use_noref(&rt->dst, jiffies);
>> +                       rt6_dst_from_metrics_check(rt);
>> +               }
>>                 read_unlock_bh(&table->tb6_lock);
>> -
>> -               rt6_dst_from_metrics_check(rt);
>> -
>>                 trace_fib6_table_lookup(net, rt, table->tb6_id, fl6);
>>                 return rt;
>>         } else if (unlikely((fl6->flowi6_flags & FLOWI_FLAG_KNOWN_NH) &&
>
> Is it intended to move rt6_dst_from_metrics_check() inside the table lock?
>

I think it doesn't really matter whether rt6_dst_from_metrics_check()
is inside the table lock or not. The code looks cleaner if we put it
inside the if (ip6_hold_safe()) {} block because we don't want to do
rt6_dst_from_metrics_check() if ip6_hold_safe() returns false.

> --yoshfuji