netdev - Re: [PATCH RFC net-next 07/10] net: dsa: mv88e6xxx: Track bridge mdb objects

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Date: Tue, 30 Apr 2024 03:59:49 +0300
From: Vladimir Oltean <olteanv@...il.com>
To: Joseph Huang <joseph.huang.2024@...il.com>
Cc: Joseph Huang <Joseph.Huang@...min.com>, netdev@...r.kernel.org,
	Andrew Lunn <andrew@...n.ch>,
	Florian Fainelli <f.fainelli@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Roopa Prabhu <roopa@...dia.com>,
	Nikolay Aleksandrov <razor@...ckwall.org>,
	Linus Lüssing <linus.luessing@...3.blue>,
	linux-kernel@...r.kernel.org, bridge@...ts.linux.dev
Subject: Re: [PATCH RFC net-next 07/10] net: dsa: mv88e6xxx: Track bridge mdb
 objects

On Mon, Apr 29, 2024 at 06:07:25PM -0400, Joseph Huang wrote:
> Something like this (some layers omitted for brevity)?
> 
>                                       +br_iterator
>                                       |  for each mdb
>                                       |    _br_switchdev_mdb_notify
> rtnl_lock                             |      without F_DEFER flag
>  |                                    |      |
>  +switchdev_port_attr_set_deferred    |      +switchdev_port_obj_notify
>    |                                  |        |
>    +dsa_port_mrouter                  |        +dsa_user_port_obj_a/d
>      |                                |          |
>      +mv88e6xxx_port_mrouter----------+          +mv88e6xxx_port_obj_a/d
>                                         |
>  +--------------------------------------+
>  |
> rtnl_unlock

At a _very_ superficial glance, I don't think you are properly
accounting for the fact that even with rtnl_lock() held, there are still
SWITCHDEV_OBJ_ID_PORT_MDB events which may be pending on the switchdev
chain. Without a switchdev_deferred_process() flush call, you won't be
getting rid of them, so when you rtnl_unlock(), they will still run.

Even worse, holding rtnl_lock() will not stop the bridge multicast layer
from modifying its br->mdb_list; only br->multicast_lock will.

So you may be better off also acquiring br->multicast_lock, and
notifying the MDB entries to the switchdev chain _with_the F_DEFER flag.

> Note that on the system I tested, each register read/write takes about 100us
> to complete. For 100's of mdb groups, this would mean that we will be
> holding rtnl lock for 10's of ms. I don't know if it's considered too long.

Not sure how this is going to be any better if the iteration over MDB
entries is done 100% in the driver, though - since its hook,
dsa_port_mrouter(), runs entirely under rtnl_lock().

Anyway, with the SWITCHDEV_F_DEFER flag, maybe the mdb object
notifications can be made to run by switchdev only a few at a time, to
give the network stack time to do other things as well.