netdev - Re: [PATCH v3 net-next 7/9] net: dsa: drop rtnl_lock from dsa_slave_switchdev_event

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Message-ID: <d33925e1-c28a-7643-9a29-f37f83957bb2@gmail.com>
Date:   Fri, 22 Oct 2021 10:37:24 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        UNGLinuxDriver@...rochip.com, DENG Qingfang <dqfext@...il.com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Hauke Mehrtens <hauke@...ke-m.de>,
        Woojung Huh <woojung.huh@...rochip.com>,
        Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        George McCollister <george.mccollister@...il.com>,
        John Crispin <john@...ozen.org>,
        Aleksander Jan Bajkowski <olek2@...pl>,
        Egil Hjelmeland <privat@...l-hjelmeland.no>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        Prasanna Vengateshan <prasanna.vengateshan@...rochip.com>,
        Ansuel Smith <ansuelsmth@...il.com>,
        Alvin Šipraga <alsi@...g-olufsen.dk>
Subject: Re: [PATCH v3 net-next 7/9] net: dsa: drop rtnl_lock from
 dsa_slave_switchdev_event_work

On 10/22/21 10:27 AM, Vladimir Oltean wrote:
> After talking with Ido Schimmel, it became clear that rtnl_lock is not
> actually required for anything that is done inside the
> SWITCHDEV_FDB_{ADD,DEL}_TO_DEVICE deferred work handlers.
> 
> The reason why it was probably added by Arkadi Sharshevsky in commit
> c9eb3e0f8701 ("net: dsa: Add support for learning FDB through
> notification") was to offer the same locking/serialization guarantees as
> .ndo_fdb_{add,del} and avoid reworking any drivers.
> 
> DSA has implemented .ndo_fdb_add and .ndo_fdb_del until commit
> b117e1e8a86d ("net: dsa: delete dsa_legacy_fdb_add and
> dsa_legacy_fdb_del") - that is to say, until fairly recently.
> 
> But those methods have been deleted, so now we are free to drop the
> rtnl_lock as well.
> 
> Note that exposing DSA switch drivers to an unlocked method which was
> previously serialized by the rtnl_mutex is a potentially dangerous
> affair. Driver writers couldn't ensure that their internal locking
> scheme does the right thing even if they wanted.
> 
> We could err on the side of paranoia and introduce a switch-wide lock
> inside the DSA framework, but that seems way overreaching. Instead, we
> could check as many drivers for regressions as we can, fix those first,
> then let this change go in once it is assumed to be fairly safe.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Reviewed-by: Florian Fainelli <f.fainelli@...il.com>
-- 
Florian