Re: [PATCH net-next 1/2] net: use a single lockdep class for the netdev instance lock

[Jakub Kicinski <kuba@kernel.org>]

On Wed, 23 Sep 2026 15:35:44 -0700 Jakub Kicinski wrote:
>  	(dev)->qdisc_tx_busylock = &qdisc_tx_busylock_key;	\
>  	lockdep_set_class(&(dev)->addr_list_lock,		\
>  			  &dev_addr_list_lock_key);		\
> -	lockdep_set_class(&(dev)->lock,				\
> -			  &dev_instance_lock_key);		\
> -	lock_set_cmp_fn(&dev->lock, netdev_lock_cmp_fn, NULL);	\
>  	for (i = 0; i < (dev)->num_tx_queues; i++)		\
>  		lockdep_set_class(&(dev)->_tx[i]._xmit_lock,	\
>  				  &qdisc_xmit_lock_key);	\
> diff --git a/net/core/dev.c b/net/core/dev.c
> index c67900354fa6..bca7b267f937 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -568,6 +568,22 @@ static inline void netdev_set_addr_lockdep_class(struct net_device *dev)
>  }
>  #endif
>  
> +#ifdef CONFIG_PROVE_LOCKING
> +static int netdev_lock_cmp_fn(const struct lockdep_map *a,
> +			      const struct lockdep_map *b)
> +{
> +	if (a == b)
> +		return 0;
> +
> +	/* Allow locking multiple devices only under rtnl_lock,
> +	 * the exact order doesn't matter.
> +	 * Note that upper devices don't lock their ops, so nesting
> +	 * mostly happens in batched device removal for now.
> +	 */
> +	return lockdep_rtnl_is_held() ? -1 : 1;
> +}
> +#endif

ugh, clashiko points out this is weakening the condition more than
I thought. Apparently lockdep_rtnl_is_held() is always UNKNOWN (-1)
when called from inside lockdep code ;( So this lockdep_rtnl_is_held()
is always true, and we would never fire a lockdep warning on instance
locks.

I need to switch to non-lockdep rtnl_is_held(), I think, and we either
bring back the classes for virt devices (but one class for all) or
we explicitly handle the virt->phys order that queue leasing needs.
Coding it up now, let me see which one is less ugly.