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Date:   Fri, 3 Mar 2017 13:55:52 +0000
From:   Igor Druzhinin <igor.druzhinin@...rix.com>
To:     Paul Durrant <Paul.Durrant@...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

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.

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/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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