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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Igor Druzhinin
> Sent: 03 March 2017 13:54
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx; xen-
> devel@xxxxxxxxxxxxxxxxxxxx
> Cc: jgross@xxxxxxxx; Wei Liu <wei.liu2@xxxxxxxxxx>
> Subject: Re: [PATCH] xen-netback: fix race condition on XenBus disconnect
> 
> On 03/03/17 09:18, Paul Durrant wrote:
> >> -----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/ne
> t/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
> >
> 
> xenvif_up() and xenvif_down() are called under rtnl_lock.
> xenvif_get_ethtool_stats() is a good candidate. I'm not sure about
> xenvif_fatal_tx_err. Is it actually called from already protected regions?
> 

IIRC it's called out of the NAPI worker so I'd hope turning carrier off would 
be sufficient. I still thing that testing for !num_queues everywhere rather 
than !queues in some places would be better, even if you don't actually need to 
rcu protect all the tests.

  Paul

> Igor
> 
> >>
> >> -          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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