Re: [PATCH net-next v4 07/12] net: hold netdev instance lock during ndo_bpf

[Jakub Kicinski <kuba@kernel.org>]

On Mon, 17 Feb 2025 18:09:43 -0800 Stanislav Fomichev wrote:
> --- a/kernel/bpf/offload.c
> +++ b/kernel/bpf/offload.c
> @@ -528,10 +528,10 @@ struct bpf_map *bpf_map_offload_map_alloc(union bpf_attr *attr)
>  		return ERR_PTR(-ENOMEM);
>  
>  	bpf_map_init_from_attr(&offmap->map, attr);
> -
>  	rtnl_lock();
> -	down_write(&bpf_devs_lock);
>  	offmap->netdev = __dev_get_by_index(net, attr->map_ifindex);
> +	netdev_lock_ops(offmap->netdev);
> +	down_write(&bpf_devs_lock);
>  	err = bpf_dev_offload_check(offmap->netdev);
>  	if (err)
>  		goto err_unlock;
> @@ -548,12 +548,14 @@ struct bpf_map *bpf_map_offload_map_alloc(union bpf_attr *attr)
>  
>  	list_add_tail(&offmap->offloads, &ondev->maps);
>  	up_write(&bpf_devs_lock);
> +	netdev_unlock_ops(offmap->netdev);
>  	rtnl_unlock();
>  
>  	return &offmap->map;
>  
>  err_unlock:
>  	up_write(&bpf_devs_lock);
> +	netdev_unlock_ops(offmap->netdev);
>  	rtnl_unlock();
>  	bpf_map_area_free(offmap);
>  	return ERR_PTR(err);

Any reason for this lock ordering? My intuition would be from biggest
to smallest, so rtnl_lock -> sem -> instance