netdev - Re: [PATCH v2 net-next] net: dsa: reference count the host mdb addresses

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Date:   Sat, 12 Dec 2020 20:28:08 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Tobias Waldekranz <tobias@...dekranz.com>
Subject: Re: [PATCH v2 net-next] net: dsa: reference count the host mdb
 addresses

> +/* DSA can directly translate this to a normal MDB add, but on the CPU port.
> + * But because multiple user ports can join the same multicast group and the
> + * bridge will emit a notification for each port, we need to add/delete the
> + * entry towards the host only once, so we reference count it.
> + */
> +static int dsa_host_mdb_add(struct dsa_port *dp,
> +			    const struct switchdev_obj_port_mdb *mdb,
> +			    struct switchdev_trans *trans)
> +{
> +	struct dsa_port *cpu_dp = dp->cpu_dp;
> +	struct dsa_switch *ds = dp->ds;
> +	struct dsa_host_addr *a;
> +	int err;
> +
> +	a = dsa_host_addr_find(&ds->host_mdb, mdb);
> +	if (a) {
> +		/* Only the commit phase is refcounted */
> +		if (switchdev_trans_ph_commit(trans))
> +			refcount_inc(&a->refcount);
> +		return 0;
> +	}
> +
> +	err = dsa_port_mdb_add(cpu_dp, mdb, trans);
> +	if (err)
> +		return err;
> +
> +	/* Only the commit phase is refcounted, so don't save this just yet */
> +	if (switchdev_trans_ph_prepare(trans))
> +		return 0;
> +
> +	a = kzalloc(sizeof(*a), GFP_KERNEL);
> +	if (!a)
> +		return -ENOMEM;

Hi Vladimir

This allocation should be done in the prepare phase, since it can
fail. You should only return catastrophic errors during the commit
phase.

So i think you can allocate it in the prepare phase, but leave the
reference count as 0. Then increment it in the commit phase.

And then make the dsa_host_mdb_del() decrement the specific refcount,
and then do a generic garbage collect for all entries with refcount of
0, to cleanup any left over failures.

   Andrew