Re: [PATCH v2 net-next 04/11] net: bridge: offload initial and final port flags through switchdev

[Ido Schimmel]

On Wed, Feb 10, 2021 at 12:51:53AM +0200, Vladimir Oltean wrote:
> On Wed, Feb 10, 2021 at 12:01:24AM +0200, Ido Schimmel wrote:
> > On Tue, Feb 09, 2021 at 10:20:45PM +0200, Vladimir Oltean wrote:
> > > On Tue, Feb 09, 2021 at 08:51:00PM +0200, Ido Schimmel wrote:
> > > > On Tue, Feb 09, 2021 at 05:19:29PM +0200, Vladimir Oltean wrote:
> > > > > So switchdev drivers operating in standalone mode should disable address
> > > > > learning. As a matter of practicality, we can reduce code duplication in
> > > > > drivers by having the bridge notify through switchdev of the initial and
> > > > > final brport flags. Then, drivers can simply start up hardcoded for no
> > > > > address learning (similar to how they already start up hardcoded for no
> > > > > forwarding), then they only need to listen for
> > > > > SWITCHDEV_ATTR_ID_PORT_BRIDGE_FLAGS and their job is basically done, no
> > > > > need for special cases when the port joins or leaves the bridge etc.
> > > >
> > > > How are you handling the case where a port leaves a LAG that is linked
> > > > to a bridge? In this case the port becomes a standalone port, but will
> > > > not get this notification.
> > >
> > > Apparently the answer to that question is "I delete the code that makes
> > > this use case work", how smart of me. Thanks.
> >
> > Not sure how you expect to interpret this.
> 
> Next patch (05/11) deletes that explicit notification from dsa_port_bridge_leave,
> function which is called from dsa_port_lag_leave too, apparently with good reason.
> 
> > > Unless you have any idea how I could move the logic into the bridge, I
> > > guess I'm stuck with DSA and all the other switchdev drivers having this
> > > forest of corner cases to deal with. At least I can add a comment so I'm
> > > not tempted to delete it next time.
> >
> > There are too many moving pieces with stacked devices. It is not only
> > LAG/bridge. In L3 you have VRFs, SVIs, macvlans etc. It might be better
> > to gracefully / explicitly not handle a case rather than pretending to
> > handle it correctly with complex / buggy code.
> >
> > For example, you should refuse to be enslaved to a LAG that already has
> > upper devices such as a bridge. You are probably not handling this
> > correctly / at all. This is easy. Just a call to
> > netdev_has_any_upper_dev().
> 
> Correct, good point, in particular this means that joining a bridged LAG
> will not get me any notifications of that LAG's CHANGEUPPER because that
> was consumed a long time ago. An equally valid approach seems to be to
> check for netdev_master_upper_dev_get_rcu in dsa_port_lag_join, and call
> dsa_port_bridge_join on the upper if that is present.

The bridge might already have a state you are not familiar with (e.g.,
FDB entry pointing to the LAG), so best to just forbid this. I think
it's fair to impose such limitations (assuming they are properly
communicated to user space) given it results in a much less
buggy/complex code to maintain.

> 
> > The reverse, during unlinking, would be to refuse unlinking if the upper
> > has uppers of its own. netdev_upper_dev_unlink() needs to learn to
> > return an error and callers such as team/bond need to learn to handle
> > it, but it seems patchable.
> 
> Again, this was treated prior to my deletion in this series and not by
> erroring out, I just really didn't think it through.
> 
> So you're saying that if we impose that all switchdev drivers restrict
> the house of cards to be constructed from the bottom up, and destructed
> from the top down, then the notification of bridge port flags can stay
> in the bridge layer?

I actually don't think it's a good idea to have this in the bridge in
any case. I understand that it makes sense for some devices where
learning, flooding, etc are port attributes, but in other devices these
can be {port,vlan} attributes and then you need to take care of them
when a vlan is added / deleted and not only when a port is removed from
the bridge. So for such devices this really won't save anything. I would
thus leave it to the lower levels to decide.