netdev - Re: [net-next 08/13] net/mlx5e: Protect tc flows hashtable with rcu

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Date:   Tue, 30 Jul 2019 12:15:42 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Saeed Mahameed <saeedm@...lanox.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Vlad Buslov <vladbu@...lanox.com>,
        Jianbo Liu <jianbol@...lanox.com>,
        Roi Dayan <roid@...lanox.com>
Subject: Re: [net-next 08/13] net/mlx5e: Protect tc flows hashtable with rcu

On Mon, Jul 29, 2019 at 7:50 PM Saeed Mahameed <saeedm@...lanox.com> wrote:
>
> From: Vlad Buslov <vladbu@...lanox.com>
>
> In order to remove dependency on rtnl lock, access to tc flows hashtable
> must be explicitly protected from concurrent flows removal.
>
> Extend tc flow structure with rcu to allow concurrent parallel access. Use
> rcu read lock to safely lookup flow in tc flows hash table, and take
> reference to it. Use rcu free for flow deletion to accommodate concurrent
> stats requests.
>
> Add new DELETED flow flag. Imlement new flow_flag_test_and_set() helper
> that is used to set a flag and return its previous value. Use it to
> atomically set the flag in mlx5e_delete_flower() to guarantee that flow can
> only be deleted once, even when same flow is deleted concurrently by
> multiple tasks.
>
> Signed-off-by: Vlad Buslov <vladbu@...lanox.com>
> Reviewed-by: Jianbo Liu <jianbol@...lanox.com>
> Reviewed-by: Roi Dayan <roid@...lanox.com>
> Signed-off-by: Saeed Mahameed <saeedm@...lanox.com>
> ---

> @@ -3492,16 +3507,32 @@ int mlx5e_delete_flower(struct net_device *dev, struct mlx5e_priv *priv,
>  {
>         struct rhashtable *tc_ht = get_tc_ht(priv, flags);
>         struct mlx5e_tc_flow *flow;
> +       int err;
>
> +       rcu_read_lock();
>         flow = rhashtable_lookup_fast(tc_ht, &f->cookie, tc_ht_params);
> -       if (!flow || !same_flow_direction(flow, flags))
> -               return -EINVAL;
> +       if (!flow || !same_flow_direction(flow, flags)) {
> +               err = -EINVAL;
> +               goto errout;
> +       }
>
> +       /* Only delete the flow if it doesn't have MLX5E_TC_FLOW_DELETED flag
> +        * set.
> +        */
> +       if (flow_flag_test_and_set(flow, DELETED)) {
> +               err = -EINVAL;
> +               goto errout;
> +       }
>         rhashtable_remove_fast(tc_ht, &flow->node, tc_ht_params);
> +       rcu_read_unlock();
>
>         mlx5e_flow_put(priv, flow);

Dereferencing flow outside rcu readside critical section? Does a build
with lockdep not complain?

>
>         return 0;
> +
> +errout:
> +       rcu_read_unlock();
> +       return err;
>  }
>
>  int mlx5e_stats_flower(struct net_device *dev, struct mlx5e_priv *priv,
> @@ -3517,8 +3548,10 @@ int mlx5e_stats_flower(struct net_device *dev, struct mlx5e_priv *priv,
>         u64 bytes = 0;
>         int err = 0;
>
> -       flow = mlx5e_flow_get(rhashtable_lookup_fast(tc_ht, &f->cookie,
> -                                                    tc_ht_params));
> +       rcu_read_lock();
> +       flow = mlx5e_flow_get(rhashtable_lookup(tc_ht, &f->cookie,
> +                                               tc_ht_params));
> +       rcu_read_unlock();
>         if (IS_ERR(flow))
>                 return PTR_ERR(flow);

Same, in code below this check?