linux-kernel - Re: [PATCH 5.15 301/917] net: dsa: flush switchdev workqueue when leaving the bridge

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Message-ID: <20211115233358.u6n44mop73le7rkw@skbuf>
Date:   Mon, 15 Nov 2021 23:33:59 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.15 301/917] net: dsa: flush switchdev workqueue when
 leaving the bridge

On Mon, Nov 15, 2021 at 05:56:36PM +0100, Greg Kroah-Hartman wrote:
> From: Vladimir Oltean <vladimir.oltean@....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@....com>
> Signed-off-by: David S. Miller <davem@...emloft.net>
> Signed-off-by: Sasha Levin <sashal@...nel.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.