Date:	Mon, 23 Jun 2014 10:05:20 +0000
From:	Paul Durrant <Paul.Durrant@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	"boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
	Wei Liu <wei.liu2@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>
Subject: RE: [PATCH net v3] xen-netback: bookkeep number of active queues in
 our own module

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@...rix.com]
> Sent: 23 June 2014 10:50
> To: xen-devel@...ts.xen.org; netdev@...r.kernel.org
> Cc: boris.ostrovsky@...cle.com; Wei Liu; Ian Campbell; Paul Durrant
> Subject: [PATCH net v3] xen-netback: bookkeep number of active queues in
> our own module
> 
> The original code uses netdev->real_num_tx_queues to bookkeep number
> of
> queues and invokes netif_set_real_num_tx_queues to set the number of
> queues. However, netif_set_real_num_tx_queues doesn't allow
> real_num_tx_queues to be smaller than 1, which means setting the number
> to 0 will not work and real_num_tx_queues is untouched.
> 
> This is bogus when xenvif_free is invoked before any number of queues is
> allocated. That function needs to iterate through all queues to free
> resources. Using the wrong number of queues results in NULL pointer
> dereference.
> 
> So we bookkeep the number of queues in xen-netback to solve this
> problem. This fixes a regression introduced by multiqueue patchset in
> 3.16-rc1.
> 
> There's another bug in original code that the real number of RX queues
> is never set. In current Xen multiqueue design, the number of TX queues
> and RX queues are in fact the same. We need to set the numbers of TX and
> RX queues to the same value.
> 
> Also remove xenvif_select_queue and leave queue selection to core
> driver, as suggested by David Miller.
> 
> Reported-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Signed-off-by: Wei Liu <wei.liu2@...rix.com>
> CC: Ian Campbell <ian.campbell@...rix.com>
> CC: Paul Durrant <paul.durrant@...rix.com>
> ---
> Change since v2:
> * improve commit message
> ---
>  drivers/net/xen-netback/common.h    |    1 +
>  drivers/net/xen-netback/interface.c |   49 ++++++++---------------------------
>  drivers/net/xen-netback/xenbus.c    |   28 ++++++++++----------
>  3 files changed, 26 insertions(+), 52 deletions(-)
> 
> diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-
> netback/common.h
> index 4dd7c4a..2532ce8 100644
> --- a/drivers/net/xen-netback/common.h
> +++ b/drivers/net/xen-netback/common.h
> @@ -222,6 +222,7 @@ struct xenvif {
> 
>  	/* Queues */
>  	struct xenvif_queue *queues;
> +	unsigned int num_queues; /* active queues, resource allocated */
> 
>  	/* Miscellaneous private stuff. */
>  	struct net_device *dev;
> diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-
> netback/interface.c
> index 852da34..9e97c7c 100644
> --- a/drivers/net/xen-netback/interface.c
> +++ b/drivers/net/xen-netback/interface.c
> @@ -137,32 +137,11 @@ static void xenvif_wake_queue_callback(unsigned
> long data)
>  	}
>  }
> 
> -static u16 xenvif_select_queue(struct net_device *dev, struct sk_buff *skb,
> -			       void *accel_priv, select_queue_fallback_t
> fallback)
> -{
> -	unsigned int num_queues = dev->real_num_tx_queues;
> -	u32 hash;
> -	u16 queue_index;
> -
> -	/* First, check if there is only one queue to optimise the
> -	 * single-queue or old frontend scenario.
> -	 */
> -	if (num_queues == 1) {
> -		queue_index = 0;
> -	} else {
> -		/* Use skb_get_hash to obtain an L4 hash if available */
> -		hash = skb_get_hash(skb);
> -		queue_index = hash % num_queues;
> -	}
> -
> -	return queue_index;
> -}
> -
>  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 = dev->real_num_tx_queues;
> +	unsigned int num_queues = vif->num_queues;
>  	u16 index;
>  	int min_slots_needed;
> 
> @@ -225,7 +204,7 @@ 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 = dev->real_num_tx_queues;
> +	unsigned int num_queues = vif->num_queues;
>  	unsigned long rx_bytes = 0;
>  	unsigned long rx_packets = 0;
>  	unsigned long tx_bytes = 0;
> @@ -256,7 +235,7 @@ out:
>  static void xenvif_up(struct xenvif *vif)
>  {
>  	struct xenvif_queue *queue = NULL;
> -	unsigned int num_queues = vif->dev->real_num_tx_queues;
> +	unsigned int num_queues = vif->num_queues;
>  	unsigned int queue_index;
> 
>  	for (queue_index = 0; queue_index < num_queues;
> ++queue_index) {
> @@ -272,7 +251,7 @@ static void xenvif_up(struct xenvif *vif)
>  static void xenvif_down(struct xenvif *vif)
>  {
>  	struct xenvif_queue *queue = NULL;
> -	unsigned int num_queues = vif->dev->real_num_tx_queues;
> +	unsigned int num_queues = vif->num_queues;
>  	unsigned int queue_index;
> 
>  	for (queue_index = 0; queue_index < num_queues;
> ++queue_index) {
> @@ -379,7 +358,7 @@ 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 = dev->real_num_tx_queues;
> +	unsigned int num_queues = vif->num_queues;
>  	int i;
>  	unsigned int queue_index;
>  	struct xenvif_stats *vif_stats;
> @@ -424,7 +403,6 @@ static const struct net_device_ops
> xenvif_netdev_ops = {
>  	.ndo_fix_features = xenvif_fix_features,
>  	.ndo_set_mac_address = eth_mac_addr,
>  	.ndo_validate_addr   = eth_validate_addr,
> -	.ndo_select_queue = xenvif_select_queue,

Ok, so we are in a situation where the frontend does not know what the backend is doing. I guess, since the negotiation of the algorithm through xenstore is completely lacking at the moment, then this is the status quo. So using the default __netdev_pick_tx() is ok.
I do think the fact that the frontend doesn't care what algorithm the backend is using needs to be made explicit though. I'll see what I can do this week.

  Paul

>  };
> 
>  struct xenvif *xenvif_alloc(struct device *parent, domid_t domid,
> @@ -438,7 +416,7 @@ struct xenvif *xenvif_alloc(struct device *parent,
> domid_t domid,
>  	snprintf(name, IFNAMSIZ - 1, "vif%u.%u", domid, handle);
>  	/* Allocate a netdev with the max. supported number of queues.
>  	 * When the guest selects the desired number, it will be updated
> -	 * via netif_set_real_num_tx_queues().
> +	 * via netif_set_real_num_*_queues().
>  	 */
>  	dev = alloc_netdev_mq(sizeof(struct xenvif), name, ether_setup,
>  			      xenvif_max_queues);
> @@ -458,11 +436,9 @@ struct xenvif *xenvif_alloc(struct device *parent,
> domid_t domid,
>  	vif->dev = dev;
>  	vif->disabled = false;
> 
> -	/* Start out with no queues. The call below does not require
> -	 * rtnl_lock() as it happens before register_netdev().
> -	 */
> +	/* Start out with no queues. */
>  	vif->queues = NULL;
> -	netif_set_real_num_tx_queues(dev, 0);
> +	vif->num_queues = 0;
> 
>  	dev->netdev_ops	= &xenvif_netdev_ops;
>  	dev->hw_features = NETIF_F_SG |
> @@ -677,7 +653,7 @@ static void xenvif_wait_unmap_timeout(struct
> xenvif_queue *queue,
>  void xenvif_disconnect(struct xenvif *vif)
>  {
>  	struct xenvif_queue *queue = NULL;
> -	unsigned int num_queues = vif->dev->real_num_tx_queues;
> +	unsigned int num_queues = vif->num_queues;
>  	unsigned int queue_index;
> 
>  	if (netif_carrier_ok(vif->dev))
> @@ -724,7 +700,7 @@ void xenvif_deinit_queue(struct xenvif_queue
> *queue)
>  void xenvif_free(struct xenvif *vif)
>  {
>  	struct xenvif_queue *queue = NULL;
> -	unsigned int num_queues = vif->dev->real_num_tx_queues;
> +	unsigned int num_queues = vif->num_queues;
>  	unsigned int queue_index;
>  	/* Here we want to avoid timeout messages if an skb can be
> legitimately
>  	 * stuck somewhere else. Realistically this could be an another vif's
> @@ -748,12 +724,9 @@ void xenvif_free(struct xenvif *vif)
>  		xenvif_deinit_queue(queue);
>  	}
> 
> -	/* Free the array of queues. The call below does not require
> -	 * rtnl_lock() because it happens after unregister_netdev().
> -	 */
> -	netif_set_real_num_tx_queues(vif->dev, 0);
>  	vfree(vif->queues);
>  	vif->queues = NULL;
> +	vif->num_queues = 0;
> 
>  	free_netdev(vif->dev);
> 
> diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-
> netback/xenbus.c
> index 96c63dc2..3d85acd 100644
> --- a/drivers/net/xen-netback/xenbus.c
> +++ b/drivers/net/xen-netback/xenbus.c
> @@ -527,9 +527,7 @@ static void connect(struct backend_info *be)
>  	/* Use the number of queues requested by the frontend */
>  	be->vif->queues = vzalloc(requested_num_queues *
>  				  sizeof(struct xenvif_queue));
> -	rtnl_lock();
> -	netif_set_real_num_tx_queues(be->vif->dev,
> requested_num_queues);
> -	rtnl_unlock();
> +	be->vif->num_queues = requested_num_queues;
> 
>  	for (queue_index = 0; queue_index < requested_num_queues;
> ++queue_index) {
>  		queue = &be->vif->queues[queue_index];
> @@ -546,9 +544,7 @@ static void connect(struct backend_info *be)
>  			 * earlier queues can be destroyed using the regular
>  			 * disconnect logic.
>  			 */
> -			rtnl_lock();
> -			netif_set_real_num_tx_queues(be->vif->dev,
> queue_index);
> -			rtnl_unlock();
> +			be->vif->num_queues = queue_index;
>  			goto err;
>  		}
> 
> @@ -561,13 +557,19 @@ static void connect(struct backend_info *be)
>  			 * and also clean up any previously initialised queues.
>  			 */
>  			xenvif_deinit_queue(queue);
> -			rtnl_lock();
> -			netif_set_real_num_tx_queues(be->vif->dev,
> queue_index);
> -			rtnl_unlock();
> +			be->vif->num_queues = queue_index;
>  			goto err;
>  		}
>  	}
> 
> +	/* Initialisation completed, tell core driver the number of
> +	 * active queues.
> +	 */
> +	rtnl_lock();
> +	netif_set_real_num_tx_queues(be->vif->dev,
> requested_num_queues);
> +	netif_set_real_num_rx_queues(be->vif->dev,
> requested_num_queues);
> +	rtnl_unlock();
> +
>  	xenvif_carrier_on(be->vif);
> 
>  	unregister_hotplug_status_watch(be);
> @@ -582,13 +584,11 @@ static void connect(struct backend_info *be)
>  	return;
> 
>  err:
> -	if (be->vif->dev->real_num_tx_queues > 0)
> +	if (be->vif->num_queues > 0)
>  		xenvif_disconnect(be->vif); /* Clean up existing queues */
>  	vfree(be->vif->queues);
>  	be->vif->queues = NULL;
> -	rtnl_lock();
> -	netif_set_real_num_tx_queues(be->vif->dev, 0);
> -	rtnl_unlock();
> +	be->vif->num_queues = 0;
>  	return;
>  }
> 
> @@ -596,7 +596,7 @@ err:
>  static int connect_rings(struct backend_info *be, struct xenvif_queue
> *queue)
>  {
>  	struct xenbus_device *dev = be->dev;
> -	unsigned int num_queues = queue->vif->dev-
> >real_num_tx_queues;
> +	unsigned int num_queues = queue->vif->num_queues;
>  	unsigned long tx_ring_ref, rx_ring_ref;
>  	unsigned int tx_evtchn, rx_evtchn;
>  	int err;
> --
> 1.7.10.4

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