From: Vladimir Oltean <olte...@gmail.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.wangc...@gmail.com>
> Signed-off-by: Vladimir Oltean <olte...@gmail.com>
Applied and queued up for v5.8 -stable, thanks.
