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

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Date: Thu, 2 May 2024 16:37:16 -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 4/30/2024 12:27 PM, Joseph Huang wrote:
> 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 (between rtnl_lock and br->multicast_lock)?

Hi Nik,

Do you know if it's safe to acquire rtnl_lock and br->multicast_lock in 
the following sequence? Is there any potential possibility for a deadlock?

rtnl_lock
spin_lock(br->multicast_lock)
spin_unlock(br->multicast_lock)
rtnl_unlock

If the operation is safe, the next question is should it be spin_lock or 
spin_lock_bh?

Thanks,
Joseph