On Tue, Jul 30, 2019 at 12:16 PM Willem de Bruijn
<willemdebruijn.ker...@gmail.com> wrote:
>
> On Mon, Jul 29, 2019 at 7:50 PM Saeed Mahameed <sae...@mellanox.com> wrote:
> >
> > From: Vlad Buslov <vla...@mellanox.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 <vla...@mellanox.com>
> > Reviewed-by: Jianbo Liu <jian...@mellanox.com>
> > Reviewed-by: Roi Dayan <r...@mellanox.com>
> > Signed-off-by: Saeed Mahameed <sae...@mellanox.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.
