Re: [PATCH net v2] net-sysfs: release queue trackers before allowing reuse

[Antoine Tenart <atenart@kernel.org>]

On Thu, Oct 01, 2026 at 02:11:06AM +0800, Chengfeng Ye wrote:
> An interrupted sysfs_rtnl_lock() can drop the last kobject reference to a
> removed TX queue without holding RTNL. netdev_queue_release() clears the
> kobject before releasing queue->dev_tracker, allowing the queue to be
> re-added while the old release still needs the shared tracker slot:
> 
>   CPU 0                               CPU 1
>   netdev_queue_release()
>     memset(kobj, 0, sizeof(*kobj))
>                                       netdev_queue_add_kobject()
>                                         state_initialized is clear
>                                         netdev_hold() installs new tracker
>     netdev_put() releases the new tracker
> 
> With CONFIG_NET_DEV_REFCNT_TRACKER enabled, the old tracker is leaked and
> the new lifetime's tracker is released prematurely. A later queue release
> then reports a double release. The numeric device references remain
> balanced.
> 
> The kernel reported:
> 
>   ref_tracker: reference already released.
>   ref_tracker: allocated in:
>    netdev_queue_update_kobjects+0x23d/0x5c0
>    netif_set_real_num_tx_queues+0x111/0x820
>    veth_set_channels+0x327/0x930
>    ethtool_set_channels+0x3ee/0x490
>   ref_tracker: freed in:
>    netdev_queue_release+0xbd/0x130
>    kobject_put+0x1f9/0x280
>    sysfs_rtnl_lock+0x18b/0x1f0
>    xps_rxqs_show+0xad/0x250
>   WARNING: lib/ref_tracker.c:322 at ref_tracker_free+0x49e/0x6d0
>   Call Trace:
>    netdev_queue_release+0xbd/0x130
>    kobject_put+0x1f9/0x280
>    netdev_queue_update_kobjects+0x3f9/0x5c0
>    netif_set_real_num_tx_queues+0x111/0x820
>    veth_set_channels+0x327/0x930
>    ethtool_set_channels+0x3ee/0x490
> 
> RX queues have the same ordering. Their removal and re-addition are
> normally serialized by RTNL, but CONFIG_DEBUG_KOBJECT_RELEASE can defer the
> release callback to workqueue context and expose the same reuse window.
> 
> Release each tracker before clearing its kobject. Pair full memory barriers
> on the release and add sides so that an add which observes
> state_initialized clear cannot install a new tracker before the old release
> has finished accessing the shared tracker slot. Keep the numeric device
> reference until after the reset so that the queue storage remains alive
> throughout the callback's accesses.
> 
> Fixes: b0b6fcfa6ad8 ("net-sysfs: remove rtnl_trylock from queue attributes")
> Cc: stable@vger.kernel.org
> Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>

Reviewed-by: Antoine Tenart <atenart@kernel.org>

> ---
> Changes in v2:
> - Add an explicit full memory barrier in the TX add path, paired with the
>   release-side barrier.
> - Apply the same tracker ordering and barrier pair to RX queues, whose
>   release callback may be delayed with CONFIG_DEBUG_KOBJECT_RELEASE.
> - Clarify the paired barrier comments.
> 
> Link: https://lore.kernel.org/r/20260926173315.2452612-1-nicoyip.dev@gmail.com/ [v1]
> 
>  net/core/net-sysfs.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
> index 352173df7578..2af972f5d3c3 100644
> --- a/net/core/net-sysfs.c
> +++ b/net/core/net-sysfs.c
> @@ -1156,8 +1156,11 @@ static void rx_queue_release(struct kobject *kobj)
>  		kvfree_rcu_mightsleep(rps_tag_to_table(tag_ptr));
>  #endif
>  
> +	netdev_tracker_free(queue->dev, &queue->dev_tracker);
> +	/* Pairs with the smp_mb() in rx_queue_add_kobject(). */
> +	smp_mb();
>  	memset(kobj, 0, sizeof(*kobj));
> -	netdev_put(queue->dev, &queue->dev_tracker);
> +	__dev_put(queue->dev);
>  }
>  
>  static const struct ns_common *rx_queue_namespace(const struct kobject *kobj)
> @@ -1230,6 +1233,9 @@ static int rx_queue_add_kobject(struct net_device *dev, int index)
>  		return -EAGAIN;
>  	}
>  
> +	/* Pairs with the smp_mb() in rx_queue_release(). */
> +	smp_mb();
> +
>  	/* Kobject_put later will trigger rx_queue_release call which
>  	 * decreases dev refcount: Take that reference here
>  	 */
> @@ -1906,8 +1912,11 @@ static void netdev_queue_release(struct kobject *kobj)
>  {
>  	struct netdev_queue *queue = to_netdev_queue(kobj);
>  
> +	netdev_tracker_free(queue->dev, &queue->dev_tracker);
> +	/* Pairs with the smp_mb() in netdev_queue_add_kobject(). */
> +	smp_mb();
>  	memset(kobj, 0, sizeof(*kobj));
> -	netdev_put(queue->dev, &queue->dev_tracker);
> +	__dev_put(queue->dev);
>  }
>  
>  static const struct ns_common *netdev_queue_namespace(const struct kobject *kobj)
> @@ -1967,6 +1976,9 @@ static int netdev_queue_add_kobject(struct net_device *dev, int index)
>  		return -EAGAIN;
>  	}
>  
> +	/* Pairs with the smp_mb() in netdev_queue_release(). */
> +	smp_mb();
> +
>  	/* Kobject_put later will trigger netdev_queue_release call
>  	 * which decreases dev refcount: Take that reference here
>  	 */
> -- 
> 2.43.0