Re: [PATCH v3 net-next 08/12] net: dsa: replay port and host-joined mdb entries when joining the bridge

Vladimir Oltean
2021-03-22T16:56:56Z

On Mon, Mar 22, 2021 at 06:35:10PM +0200, Nikolay Aleksandrov wrote:
> > + hlist_for_each_entry(mp, &br->mdb_list, mdb_node) {
>
> You cannot walk over these lists without the multicast lock or RCU. RTNL is not
> enough because of various timers and leave messages that can alter both the mdb_list
> and the port group lists. I'd prefer RCU to avoid blocking the bridge mcast.

The trouble is that I need to emulate the calling context that is
provided to SWITCHDEV_OBJ_ID_HOST_MDB and SWITCHDEV_OBJ_ID_PORT_MDB, and
that means blocking context.

So if I hold rcu_read_lock(), I need to queue up the mdb entries, and
notify the driver only after I leave the RCU critical section. The
memory footprint may temporarily blow up.

In fact this is what I did in v1:
https://patchwork.kernel.org/project/netdevbpf/patch/20210224114350.2791260-15-olteanv@gmail.com/

I just figured I could get away with rtnl_mutex protection, but it looks
like I can't. So I guess you prefer my v1?