Re: [PATCH net] net: Lock lower level devices when updating features

[Stanislav Fomichev <stfomichev@gmail.com>]

On 05/06, Cosmin Ratiu wrote:
> On Tue, 2025-05-06 at 10:12 -0700, Stanislav Fomichev wrote:
> > On 05/06, Cosmin Ratiu wrote:
> > > __netdev_update_features() expects the netdevice to be ops-locked,
> > > but
> > > it gets called recursively on the lower level netdevices to sync
> > > their
> > > features, and nothing locks those.
> > > 
> > > This commit fixes that, with the assumption that it shouldn't be
> > > possible
> > > for both higher-level and lover-level netdevices to require the
> > > instance
> > > lock, because that would lead to lock dependency warnings.
> > > 
> > > Without this, playing with higher level (e.g. vxlan) netdevices on
> > > top
> > > of netdevices with instance locking enabled can run into issues:
> > 
> > Mentioning vxlan is a bit confusing here; it shouldn't let you flip
> > lro (I
> > think). Which upper are you testing against?
> 
> It is vxlan, but LRO is just a red herring in this case, 
> mlx5e_set_features calls every feature handler in turn, and this is
> just the example I picked from the sea of stack traces.
> 
> > 
> > Trying to understand if we can cover this case in the selftests.
> > netdevsim also doesn't expose F_LRO feature... (yet?)
> 
> I see you found a way with teaming, but I think in essence a sequence
> of commands that makes __netdev_update_features call itself recursively
> once on the lower dev will trigger the netdev_ops_assert_locked on the
> lower dev.

Right, but netdev_sync_lower_features calls lower's __netdev_update_features
only for NETIF_F_UPPER_DISABLES. So it doesn't propagate all features,
only LRO AFAICT.