netdev - Re: [PATCH net-next v2 1/1] geneve: add rtnl changelink support

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Date:   Thu, 20 Jul 2017 10:58:13 -0700
From:   Pravin Shelar <pshelar@....org>
To:     Girish Moodalbail <girish.moodalbail@...cle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 1/1] geneve: add rtnl changelink support

On Thu, Jul 20, 2017 at 10:42 AM, Girish Moodalbail
<girish.moodalbail@...cle.com> wrote:
> Hello Pravin,
>
>
>>> +/* Quiesces the geneve device data path for both TX and RX. */
>>> +static inline void geneve_quiesce(struct geneve_dev *geneve,
>>> +                                 struct geneve_sock **gs4,
>>> +                                 struct geneve_sock **gs6)
>>> +{
>>> +       *gs4 = rtnl_dereference(geneve->sock4);
>>> +       rcu_assign_pointer(geneve->sock4, NULL);
>>> +
>>> +#if IS_ENABLED(CONFIG_IPV6)
>>> +       *gs6 = rtnl_dereference(geneve->sock6);
>>> +       rcu_assign_pointer(geneve->sock6, NULL);
>>> +#else
>>> +       *gs6 = NULL;
>>> +#endif
>>> +       synchronize_net();
>>> +}
>>> +
>>> +/* Resumes the geneve device data path for both TX and RX. */
>>> +static inline void geneve_unquiesce(struct geneve_dev *geneve,
>>> +                                   struct geneve_sock *gs4,
>>> +                                   struct geneve_sock __maybe_unused
>>> *gs6)
>>> +{
>>> +       rcu_assign_pointer(geneve->sock4, gs4);
>>> +#if IS_ENABLED(CONFIG_IPV6)
>>> +       rcu_assign_pointer(geneve->sock6, gs6);
>>> +#endif
>>> +       synchronize_net();
>>> +}
>>> +
>>> +static int geneve_changelink(struct net_device *dev, struct nlattr
>>> *tb[],
>>> +                            struct nlattr *data[],
>>> +                            struct netlink_ext_ack *extack)
>>> +{
>>> +       struct geneve_dev *geneve = netdev_priv(dev);
>>> +       struct geneve_sock *gs4, *gs6;
>>> +       struct ip_tunnel_info info;
>>> +       bool metadata;
>>> +       bool use_udp6_rx_checksums;
>>> +       int err;
>>> +
>>> +       /* If the geneve device is configured for metadata (or externally
>>> +        * controlled, for example, OVS), then nothing can be changed.
>>> +        */
>>> +       if (geneve->collect_md)
>>> +               return -EOPNOTSUPP;
>>> +
>>> +       /* Start with the existing info. */
>>> +       memcpy(&info, &geneve->info, sizeof(info));
>>> +       metadata = geneve->collect_md;
>>> +       use_udp6_rx_checksums = geneve->use_udp6_rx_checksums;
>>> +       err = geneve_nl2info(dev, tb, data, &info, &metadata,
>>> +                            &use_udp6_rx_checksums, true);
>>> +       if (err)
>>> +               return err;
>>> +
>>> +       if (!geneve_dst_addr_equal(&geneve->info, &info))
>>> +               dst_cache_reset(&info.dst_cache);
>>> +
>>> +       geneve_quiesce(geneve, &gs4, &gs6);
>>> +       geneve->info = info;
>>> +       geneve->collect_md = metadata;
>>> +       geneve->use_udp6_rx_checksums = use_udp6_rx_checksums;
>>> +       geneve_unquiesce(geneve, gs4, gs6);
>>> +
>>
>> This is nice trick. But it adds check for the socket in datapath. did
>> you explore updating entire device state in single atomic transaction?
>
>
> I did explore, however what I have now seemed like a more concise method to
> perform changelink operation atomically w.r.t the datapath.
>
> That said, there is one thing I could do. Today we already check for the
> socket in datapath like below:
>
> (a) ipv4 datapath today:
> ========================
>
>   geneve_xmit_skb(...)
>   |
>   +->geneve_get_v4_rt(....)
>      |
>      +---> if (!rcu_dereference(geneve->sock4))
>                 return ERR_PTR(-EIO);
>
> (b) ipv4 datapath with my current patch:
> ========================================
>
>   geneve_xmit_skb(...)
>   |
>   +->if (!rcu_dereference(geneve->sock4))
>   |      return -EIO;
>   |
>   +->geneve_get_v4_rt(....)
>      |
>      +---> if (!rcu_dereference(geneve->sock4))
>                 return ERR_PTR(-EIO);
>
> Perhaps, I could piggyback on the already existing check for NULL socket in
> geneve_get_v4_rt() and avoid the additional check I have added in datapath.
> (The situation is same for IPv6)
>
Sounds good to me. But I would like to see comment in code explaining
it bit since this is not standard RCU usage pattern.