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Date:   Tue, 08 Sep 2020 19:43:04 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     olteanv@...il.com
Cc:     kuba@...nel.org, f.fainelli@...il.com, vivien.didelot@...il.com,
        andrew@...n.ch, xiyou.wangcong@...il.com, ap420073@...il.com,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2 net] net: dsa: link interfaces with the DSA master
 to get rid of lockdep warnings

From: Vladimir Oltean <olteanv@...il.com>
Date: Tue,  8 Sep 2020 02:48:42 +0300

> Since commit 845e0ebb4408 ("net: change addr_list_lock back to static
> key"), cascaded DSA setups (DSA switch port as DSA master for another
> DSA switch port) are emitting this lockdep warning:
 ...
> Since DSA never made use of the netdev API for describing links between
> upper devices and lower devices, the dev->lower_level value of a DSA
> switch interface would be 1, which would warn when it is a DSA master.
> 
> We can use netdev_upper_dev_link() to describe the relationship between
> a DSA slave and a DSA master. To be precise, a DSA "slave" (switch port)
> is an "upper" to a DSA "master" (host port). The relationship is "many
> uppers to one lower", like in the case of VLAN. So, for that reason, we
> use the same function as VLAN uses.
> 
> There might be a chance that somebody will try to take hold of this
> interface and use it immediately after register_netdev() and before
> netdev_upper_dev_link(). To avoid that, we do the registration and
> linkage while holding the RTNL, and we use the RTNL-locked cousin of
> register_netdev(), which is register_netdevice().
> 
> Since this warning was not there when lockdep was using dynamic keys for
> addr_list_lock, we are blaming the lockdep patch itself. The network
> stack _has_ been using static lockdep keys before, and it _is_ likely
> that stacked DSA setups have been triggering these lockdep warnings
> since forever, however I can't test very old kernels on this particular
> stacked DSA setup, to ensure I'm not in fact introducing regressions.
> 
> Fixes: 845e0ebb4408 ("net: change addr_list_lock back to static key")
> Suggested-by: Cong Wang <xiyou.wangcong@...il.com>
> Signed-off-by: Vladimir Oltean <olteanv@...il.com>

Applied and queued up for v5.8 -stable, thanks.

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