Re: [Xen-devel] [PATCH] xen-netback: fix race condition on XenBus disconnect

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Igor Druzhinin [mailto:igor.druzhinin@xxxxxxxxxx]
> Sent: 02 March 2017 22:57
> To: netdev@xxxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; jgross@xxxxxxxx; Wei Liu
> <wei.liu2@xxxxxxxxxx>; Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> Subject: [PATCH] xen-netback: fix race condition on XenBus disconnect
> 
> In some cases during XenBus disconnect event handling and subsequent
> queue resource release there may be some TX handlers active on
> other processors. Use RCU in order to synchronize with them.
> 
> Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> ---
>  drivers/net/xen-netback/interface.c | 13 ++++++++-----
>  drivers/net/xen-netback/xenbus.c    | 17 +++++++----------
>  2 files changed, 15 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-
> netback/interface.c
> index a2d32676..32e2cc6 100644
> --- a/drivers/net/xen-netback/interface.c
> +++ b/drivers/net/xen-netback/interface.c
> @@ -164,7 +164,7 @@ static int xenvif_start_xmit(struct sk_buff *skb, struct
> net_device *dev)
>  {
>       struct xenvif *vif = netdev_priv(dev);
>       struct xenvif_queue *queue = NULL;
> -     unsigned int num_queues = vif->num_queues;

Do you not need an rcu_read_lock() around this and use of the num_queues value 
(as you have below)?

> +     unsigned int num_queues = rcu_dereference(vif)->num_queues;
>       u16 index;
>       struct xenvif_rx_cb *cb;
> 
> @@ -221,18 +221,21 @@ static struct net_device_stats
> *xenvif_get_stats(struct net_device *dev)
>  {
>       struct xenvif *vif = netdev_priv(dev);
>       struct xenvif_queue *queue = NULL;
> +     unsigned int num_queues;
>       u64 rx_bytes = 0;
>       u64 rx_packets = 0;
>       u64 tx_bytes = 0;
>       u64 tx_packets = 0;
>       unsigned int index;
> 
> -     spin_lock(&vif->lock);
> -     if (vif->queues == NULL)
> +     rcu_read_lock();
> +
> +     num_queues = rcu_dereference(vif)->num_queues;
> +     if (num_queues < 1)
>               goto out;
> 
>       /* Aggregate tx and rx stats from each queue */
> -     for (index = 0; index < vif->num_queues; ++index) {
> +     for (index = 0; index < num_queues; ++index) {
>               queue = &vif->queues[index];
>               rx_bytes += queue->stats.rx_bytes;
>               rx_packets += queue->stats.rx_packets;
> @@ -241,7 +244,7 @@ static struct net_device_stats
> *xenvif_get_stats(struct net_device *dev)
>       }
> 
>  out:
> -     spin_unlock(&vif->lock);
> +     rcu_read_unlock();
> 
>       vif->dev->stats.rx_bytes = rx_bytes;
>       vif->dev->stats.rx_packets = rx_packets;
> diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-
> netback/xenbus.c
> index d2d7cd9..76efb01 100644
> --- a/drivers/net/xen-netback/xenbus.c
> +++ b/drivers/net/xen-netback/xenbus.c
> @@ -495,26 +495,23 @@ static void backend_disconnect(struct
> backend_info *be)
>       struct xenvif *vif = be->vif;
> 
>       if (vif) {
> +             unsigned int num_queues = vif->num_queues;
>               unsigned int queue_index;
> -             struct xenvif_queue *queues;
> 
>               xen_unregister_watchers(vif);
>  #ifdef CONFIG_DEBUG_FS
>               xenvif_debugfs_delif(vif);
>  #endif /* CONFIG_DEBUG_FS */
>               xenvif_disconnect_data(vif);
> -             for (queue_index = 0;
> -                  queue_index < vif->num_queues;
> -                  ++queue_index)
> -                     xenvif_deinit_queue(&vif->queues[queue_index]);
> 
> -             spin_lock(&vif->lock);
> -             queues = vif->queues;
>               vif->num_queues = 0;
> -             vif->queues = NULL;
> -             spin_unlock(&vif->lock);
> +             synchronize_net();

So, num_queues is your RCU protected value, rather than the queues pointer, in 
which case I think you probably need to change code such as

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/drivers/net/xen-netback/netback.c?id=refs/tags/v4.10#n216

to be gated on num_queues.

Also shouldn't xenvif_up(), xenvif_down() and xenvif_get_ethtool_stats() not be 
using rcu_read_lock() and rcu_dereference() of num_queues as well?

  Paul

> 
> -             vfree(queues);
> +             for (queue_index = 0; queue_index < num_queues;
> ++queue_index)
> +                     xenvif_deinit_queue(&vif->queues[queue_index]);
> +
> +             vfree(vif->queues);
> +             vif->queues = NULL;
> 
>               xenvif_disconnect_ctrl(vif);
>       }
> --
> 1.8.3.1


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