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

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Date:   Mon, 22 Mar 2021 19:00:03 +0200
From:   Nikolay Aleksandrov <nikolay@...dia.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Tobias Waldekranz <tobias@...dekranz.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Roopa Prabhu <roopa@...dia.com>, Jiri Pirko <jiri@...nulli.us>,
        Ido Schimmel <idosch@...sch.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        UNGLinuxDriver@...rochip.com, Ivan Vecera <ivecera@...hat.com>,
        linux-omap@...r.kernel.org,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH v3 net-next 08/12] net: dsa: replay port and host-joined
 mdb entries when joining the bridge

On 22/03/2021 18:56, Vladimir Oltean wrote:
> 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?
> 

Indeed, if you need a blocking context then you'd have to go with v1.