Re: [Xen-devel] [PATCH net v2] xen-netback: fix race condition on XenBus disconnect

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Igor Druzhinin [mailto:igor.druzhinin@xxxxxxxxxx]
> Sent: 03 March 2017 20:23
> 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 net v2] 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>
> ---
> v2:
>  * Add protection for xenvif_get_ethtool_stats
>  * Additional comments and fixes
> ---
>  drivers/net/xen-netback/interface.c | 29 ++++++++++++++++++++++-------
>  drivers/net/xen-netback/netback.c   |  2 +-
>  drivers/net/xen-netback/xenbus.c    | 20 ++++++++++----------
>  3 files changed, 33 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-
> netback/interface.c
> index a2d32676..266b7cd 100644
> --- a/drivers/net/xen-netback/interface.c
> +++ b/drivers/net/xen-netback/interface.c
> @@ -164,13 +164,17 @@ 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;
> +     unsigned int num_queues;
>       u16 index;
>       struct xenvif_rx_cb *cb;
> 
>       BUG_ON(skb->dev != dev);
> 
> -     /* Drop the packet if queues are not set up */
> +     /* Drop the packet if queues are not set up.
> +      * This handler should be called inside an RCU read section
> +      * so we don't need to enter it here explicitly.
> +      */
> +     num_queues = rcu_dereference(vif)->num_queues;
>       if (num_queues < 1)
>               goto drop;
> 
> @@ -221,18 +225,21 @@ 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;

Is this if clause worth it? All it does is jump over the for loop, which would 
not be executed anyway, since the initial test (0 < 0) would fail.

> 
>       /* 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 +248,7 @@ 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;
> @@ -377,10 +384,16 @@ static void xenvif_get_ethtool_stats(struct
> net_device *dev,
>                                    struct ethtool_stats *stats, u64 * data)
>  {
>       struct xenvif *vif = netdev_priv(dev);
> -     unsigned int num_queues = vif->num_queues;
> +     unsigned int num_queues;
>       int i;
>       unsigned int queue_index;
> 
> +     rcu_read_lock();
> +
> +     num_queues = rcu_dereference(vif)->num_queues;
> +     if (num_queues < 1)
> +             goto out;
> +

You have introduced a semantic change with the above if clause. The 
xenvif_stats array was previously zeroed if num_queues < 1. It appears that 
ethtool does actually allocate a zeroed array to pass in here, but I wonder 
whether it is still safer to have this function zero it anyway. 

>       for (i = 0; i < ARRAY_SIZE(xenvif_stats); i++) {
>               unsigned long accum = 0;
>               for (queue_index = 0; queue_index < num_queues;
> ++queue_index) {
> @@ -389,6 +402,8 @@ static void xenvif_get_ethtool_stats(struct
> net_device *dev,
>               }
>               data[i] = accum;
>       }
> +out:
> +     rcu_read_unlock();
>  }
> 
>  static void xenvif_get_strings(struct net_device *dev, u32 stringset, u8 *
> data)
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
> netback/netback.c
> index f9bcf4a..62fa74d 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -214,7 +214,7 @@ static void xenvif_fatal_tx_err(struct xenvif *vif)
>       netdev_err(vif->dev, "fatal error; disabling device\n");
>       vif->disabled = true;
>       /* Disable the vif from queue 0's kthread */
> -     if (vif->queues)
> +     if (vif->num_queues > 0)

num_queues is unsigned so this check should not be > 0. It would be better 
simply to do:

if (vif->num_queues)

  Paul

>               xenvif_kick_thread(&vif->queues[0]);
>  }
> 
> diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-
> netback/xenbus.c
> index d2d7cd9..a56d3ea 100644
> --- a/drivers/net/xen-netback/xenbus.c
> +++ b/drivers/net/xen-netback/xenbus.c
> @@ -495,26 +495,26 @@ 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;
> +             /* At this point some of the handlers may still be active
> +              * so we need to have additional synchronization here.
> +              */
>               vif->num_queues = 0;
> -             vif->queues = NULL;
> -             spin_unlock(&vif->lock);
> +             synchronize_net();
> 
> -             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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