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

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Message-ID: <aBpRX_afG5X_rT_J@mini-arch>
Date: Tue, 6 May 2025 11:13:51 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Cosmin Ratiu <cratiu@...dia.com>
Cc: "davem@...emloft.net" <davem@...emloft.net>,
	"kuba@...nel.org" <kuba@...nel.org>,
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
	Dragos Tatulea <dtatulea@...dia.com>,
	"sdf@...ichev.me" <sdf@...ichev.me>,
	"pabeni@...hat.com" <pabeni@...hat.com>,
	"jiri@...nulli.us" <jiri@...nulli.us>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	Saeed Mahameed <saeedm@...dia.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net: Lock lower level devices when updating features

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.