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

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Message-ID: <8f700330f22b741ad72b398ff30a4468c2cb67e9.camel@nvidia.com>
Date: Tue, 6 May 2025 17:47:26 +0000
From: Cosmin Ratiu <cratiu@...dia.com>
To: "stfomichev@...il.com" <stfomichev@...il.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 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.

Thanks for the review!

Cosmin.