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    SubjectRe: [PATCH 5.15 301/917] net: dsa: flush switchdev workqueue when leaving the bridge
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    On Mon, Nov 15, 2021 at 05:56:36PM +0100, Greg Kroah-Hartman wrote:
    > From: Vladimir Oltean <vladimir.oltean@nxp.com>
    >
    > [ Upstream commit d7d0d423dbaa73fd0506e25971dfdab6bf185d00 ]
    >
    > DSA is preparing to offer switch drivers an API through which they can
    > associate each FDB entry with a struct net_device *bridge_dev. This can
    > be used to perform FDB isolation (the FDB lookup performed on the
    > ingress of a standalone, or bridged port, should not find an FDB entry
    > that is present in the FDB of another bridge).
    >
    > In preparation of that work, DSA needs to ensure that by the time we
    > call the switch .port_fdb_add and .port_fdb_del methods, the
    > dp->bridge_dev pointer is still valid, i.e. the port is still a bridge
    > port.
    >
    > This is not guaranteed because the SWITCHDEV_FDB_{ADD,DEL}_TO_DEVICE API
    > requires drivers that must have sleepable context to handle those events
    > to schedule the deferred work themselves. DSA does this through the
    > dsa_owq.
    >
    > It can happen that a port leaves a bridge, del_nbp() flushes the FDB on
    > that port, SWITCHDEV_FDB_DEL_TO_DEVICE is notified in atomic context,
    > DSA schedules its deferred work, but del_nbp() finishes unlinking the
    > bridge as a master from the port before DSA's deferred work is run.
    >
    > Fundamentally, the port must not be unlinked from the bridge until all
    > FDB deletion deferred work items have been flushed. The bridge must wait
    > for the completion of these hardware accesses.
    >
    > An attempt has been made to address this issue centrally in switchdev by
    > making SWITCHDEV_FDB_DEL_TO_DEVICE deferred (=> blocking) at the switchdev
    > level, which would offer implicit synchronization with del_nbp:
    >
    > https://patchwork.kernel.org/project/netdevbpf/cover/20210820115746.3701811-1-vladimir.oltean@nxp.com/
    >
    > but it seems that any attempt to modify switchdev's behavior and make
    > the events blocking there would introduce undesirable side effects in
    > other switchdev consumers.
    >
    > The most undesirable behavior seems to be that
    > switchdev_deferred_process_work() takes the rtnl_mutex itself, which
    > would be worse off than having the rtnl_mutex taken individually from
    > drivers which is what we have now (except DSA which has removed that
    > lock since commit 0faf890fc519 ("net: dsa: drop rtnl_lock from
    > dsa_slave_switchdev_event_work")).
    >
    > So to offer the needed guarantee to DSA switch drivers, I have come up
    > with a compromise solution that does not require switchdev rework:
    > we already have a hook at the last moment in time when the bridge is
    > still an upper of ours: the NETDEV_PRECHANGEUPPER handler. We can flush
    > the dsa_owq manually from there, which makes all FDB deletions
    > synchronous.
    >
    > Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
    > Signed-off-by: David S. Miller <davem@davemloft.net>
    > Signed-off-by: Sasha Levin <sashal@kernel.org>
    > ---
    > net/dsa/port.c | 2 ++
    > 1 file changed, 2 insertions(+)
    >
    > diff --git a/net/dsa/port.c b/net/dsa/port.c
    > index 616330a16d319..3947537ed46ba 100644
    > --- a/net/dsa/port.c
    > +++ b/net/dsa/port.c
    > @@ -380,6 +380,8 @@ void dsa_port_pre_bridge_leave(struct dsa_port *dp, struct net_device *br)
    > switchdev_bridge_port_unoffload(brport_dev, dp,
    > &dsa_slave_switchdev_notifier,
    > &dsa_slave_switchdev_blocking_notifier);
    > +
    > + dsa_flush_workqueue();
    > }
    >
    > void dsa_port_bridge_leave(struct dsa_port *dp, struct net_device *br)
    > --
    > 2.33.0
    >
    >
    >

    This patch represents preparation work for a new feature. Unless it
    constitutes a dependency for some other bugfix patches (which I doubt),
    my suggestion is to not backport it. Thanks.
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