netdev - Re: [PATCH net v2] net: Lock lower level devices when updating features

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Message-ID: <aB4ZeKV8m3GKL9qc@mini-arch>
Date: Fri, 9 May 2025 08:04:24 -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>,
	Tariq Toukan <tariqt@...dia.com>,
	Dragos Tatulea <dtatulea@...dia.com>,
	"sdf@...ichev.me" <sdf@...ichev.me>,
	"pabeni@...hat.com" <pabeni@...hat.com>,
	"horms@...nel.org" <horms@...nel.org>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	Saeed Mahameed <saeedm@...dia.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net v2] net: Lock lower level devices when updating
 features

On 05/08, Cosmin Ratiu wrote:
> On Thu, 2025-05-08 at 09:12 -0700, Stanislav Fomichev wrote:
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -10454,7 +10454,9 @@ static void
> > > netdev_sync_lower_features(struct net_device *upper,
> > >  			netdev_dbg(upper, "Disabling feature %pNF
> > > on lower dev %s.\n",
> > >  				   &feature, lower->name);
> > >  			lower->wanted_features &= ~feature;
> > > +			netdev_lock_ops(lower);
> > >  			__netdev_update_features(lower);
> > > +			netdev_unlock_ops(lower);
> > >  
> > >  			if (unlikely(lower->features & feature))
> > >  				netdev_WARN(upper, "failed to
> > > disable %pNF on %s!\n",
> > 
> > Any reason not to cover the whole section under the if()? For
> > example,
> > looking at netdev_features_change, most of its invocations are under
> > the
> > lock, so keeping the lock around it might help with consistency (and
> > we can clarify it as such in
> > Documentation/networking/netdevices.rst).
> > Plus, wanted_features is already sort of ops-protected (looking at
> > netif_disable_lro+dev_disable_lro).
> 
> The critical section could be extended for the whole if, but there are
> a lot of netdev_features_change() calls in many drivers, which I am not
> sure are ops protected. So I'd be reluctant to state that
> NETDEV_FEAT_CHANGE is ops-protected in
> Documentation/networking/netdevices.rst, even though all core
> invocations would be made with the ops lock held.

Ack, I don't think the calls in drivers/ matter, none of these are
ops-protected drivers, but we can do that separately.