Re: [PATCH v3 net-next 04/11] net: bridge: vlan: notify switchdev only when something changed

On 15/02/2022 19:02, Vladimir Oltean wrote:

Currently, when a VLAN entry is added multiple times in a row to a
bridge port, nbp_vlan_add() calls br_switchdev_port_vlan_add() each
time, even if the VLAN already exists and nothing about it has changed:

bridge vlan add dev lan12 vid 100 master static

Similarly, when a VLAN is added multiple times in a row to a bridge,
br_vlan_add_existing() doesn't filter at all the calls to
br_switchdev_port_vlan_add():

bridge vlan add dev br0 vid 100 self

This behavior makes driver-level accounting of VLANs impossible, since
it is enough for a single deletion event to remove a VLAN, but the
addition event can be emitted an unlimited number of times.

The cause for this can be identified as follows: we rely on
__vlan_add_flags() to retroactively tell us whether it has changed
anything about the VLAN flags or VLAN group pvid. So we'd first have to
call __vlan_add_flags() before calling br_switchdev_port_vlan_add(), in
order to have access to the "bool *changed" information. But we don't
want to change the event ordering, because we'd have to revert the
struct net_bridge_vlan changes we've made if switchdev returns an error.

So to solve this, we need another function that tells us whether any
change is going to occur in the VLAN or VLAN group, _prior_ to calling
__vlan_add_flags().

Split __vlan_add_flags() into a precommit and a commit stage, and rename
it to __vlan_flags_update(). The precommit stage,
__vlan_flags_would_change(), will determine whether there is any reason
to notify switchdev due to a change of flags (note: the BRENTRY flag
transition from false to true is treated separately: as a new switchdev
entry, because we skipped notifying the master VLAN when it wasn't a
brentry yet, and therefore not as a change of flags).

With this lookahead/precommit function in place, we can avoid notifying
switchdev if nothing changed for the VLAN and VLAN group.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
---
v2->v3:
- create precommit and commit wrappers around __vlan_add_flags().
- special-case the BRENTRY transition from false to true, instead of
  treating it as a change of flags and letting drivers figure out that
  it really isn't.
- avoid setting *changed unless we know that functions will not error
  out later.

v1->v2:
- drop the br_vlan_restore_existing() approach, just figure out ahead of
  time what will change.

Thanks for seeing this through.

Signed-off-by: Nikolay Aleksandrov <redacted>