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Date:   Fri, 3 Mar 2017 13:57:21 +0000
From:   Paul Durrant <Paul.Durrant@...rix.com>
To:     Igor Druzhinin <igor.druzhinin@...rix.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
CC:     "jgross@...e.com" <jgross@...e.com>, Wei Liu <wei.liu2@...rix.com>
Subject: RE: [PATCH] xen-netback: fix race condition on XenBus disconnect

> -----Original Message-----
> From: Igor Druzhinin
> Sent: 03 March 2017 13:56
> To: Paul Durrant <Paul.Durrant@...rix.com>; netdev@...r.kernel.org; xen-
> devel@...ts.xenproject.org
> Cc: jgross@...e.com; Wei Liu <wei.liu2@...rix.com>
> 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@...rix.com]
> >> Sent: 02 March 2017 22:57
> >> To: netdev@...r.kernel.org; xen-devel@...ts.xenproject.org
> >> Cc: Paul Durrant <Paul.Durrant@...rix.com>; jgross@...e.com; Wei Liu
> >> <wei.liu2@...rix.com>; Igor Druzhinin <igor.druzhinin@...rix.com>
> >> 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@...rix.com>
> >> ---
> >>  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)?
> 
> Huh, missed this one. Point is that xenvif_start_xmit is already in RCU
> read section.
> 

Ok. Probably worth a comment then since the rcu_deref looks wrong out on its own like that.

  Paul

> Igor
> 
> >
> >> +	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
> >
> >>
> >> -		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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