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

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Date:   Tue, 30 Jul 2019 12:37:55 -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 Tue, Jul 30, 2019 at 12:16 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> 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?

Eh no, it won't. The surprising part to me was to use a readside
critical section when performing a write action on an RCU ptr. The
DELETED flag ensures that multiple writers will not compete to call
rhashtable_remove_fast. rcu_read_lock is a common pattern to do
rhashtable lookup + delete.

>
> >
> >         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?

Never mind, sorry. I missed that this took a reference on the ptr
returned from rhashtable_lookup.