netdev - Re: [PATCH v3 net-next 07/11] net: prep switchdev drivers for concurrent SWITCHDEV_ATTR_ID_PORT_BRIDGE

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Date:   Wed, 10 Feb 2021 15:34:06 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     olteanv@...il.com
Cc:     kuba@...nel.org, andrew@...n.ch, vivien.didelot@...il.com,
        f.fainelli@...il.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bridge@...ts.linux-foundation.org,
        roopa@...dia.com, nikolay@...dia.com, jiri@...nulli.us,
        idosch@...sch.org, claudiu.manoil@....com,
        alexandre.belloni@...tlin.com, UNGLinuxDriver@...rochip.com,
        vkochan@...vell.com, tchornyi@...vell.com,
        grygorii.strashko@...com, ioana.ciornei@....com,
        ivecera@...hat.com, linux-omap@...r.kernel.org
Subject: Re: [PATCH v3 net-next 07/11] net: prep switchdev drivers for
 concurrent SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS

From: Vladimir Oltean <olteanv@...il.com>
Date: Wed, 10 Feb 2021 11:14:41 +0200

> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> Because the bridge will start offloading SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS
> while not serialized by any lock such as the br->lock spinlock, existing
> drivers that treat that attribute and cache the brport flags might no
> longer work correctly.
> 
> The issue is that the brport flags are a single unsigned long bitmask,
> and the bridge only guarantees the validity of the changed bits, not the
> full state. So when offloading two concurrent switchdev attributes, such
> as one for BR_LEARNING and another for BR_FLOOD, it might happen that
> the flags having BR_FLOOD are written into the cached value, and this in
> turn disables the BR_LEARNING bit which was set previously.
> 
> We can fix this across the board by keeping individual boolean variables
> for each brport flag. Note that mlxsw and prestera were setting the
> BR_LEARNING_SYNC flag too, but that appears to be just dead code, so I
> removed it.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>


This needs updating because, as discussed, there is no race.