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

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Date: Tue, 30 Apr 2024 12:27:56 -0400
From: Joseph Huang <joseph.huang.2024@...il.com>
To: Vladimir Oltean <olteanv@...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 4/29/2024 8:59 PM, Vladimir Oltean wrote:
> 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.

Like this?

                                       +br_iterator(dsa_cb)
                                       |  lock br->multicask_lock
                                       |  for each mdb
                                       |    br_switchdev_mdb_notify
rtnl_lock                             |      |
  |                                    |      +switchdev_port_obj_._defer
  +switchdev_port_attr_set_deferred    |  unlock br->multicast_lock
    |                                  |
    +dsa_port_mrouter                  |
      |                                |
      +mv88e6xxx_port_mrouter----------+
                                         |
  +--------------------------------------+
  |
rtnl_unlock

(potential task change)

rtnl_lock
  |
  +switchdev_deferred_process
  | flush all queued dfitems in queuing order
  |
rtnl_unlock

I'm not that familiar with the bridge code, but is there any concern 
with potential deadlock here (bewteen rtnl_lock and br->multicast_lock)?

> 
>> 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.