Subject: Re: [net-next rfc V3 8/9] macvtap: add TUNSETQUEUE ioctl

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 6 Jun 2013 10:23:37 +0300

On Thu, Jun 06, 2013 at 11:16:51AM +0800, Jason Wang wrote:
> On 06/05/2013 06:59 PM, Michael S. Tsirkin wrote:
> > On Wed, Jun 05, 2013 at 02:36:31PM +0800, Jason Wang wrote:
> >> This patch adds TUNSETQUEUE ioctl to let userspace can temporarily disable or
> >> enable a queue of macvtap. This is used to be compatible at API layer of tuntap
> >> to simplify the userspace to manage the queues. This is done through introducing
> >> a linked list to track all taps while using vlan->taps array to only track
> >> active taps.
> >>
> >> Signed-off-by: Jason Wang <[email-masked]>
> >> ---
> >>  drivers/net/macvtap.c      |  133 +++++++++++++++++++++++++++++++++++++++-----
> >>  include/linux/if_macvlan.h |    4 +
> >>  2 files changed, 122 insertions(+), 15 deletions(-)
> >>
> >> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> >> index 14764cc..355e6ad 100644
> >> --- a/drivers/net/macvtap.c
> >> +++ b/drivers/net/macvtap.c
> >> @@ -45,6 +45,8 @@ struct macvtap_queue {
> >>  	struct file *file;
> >>  	unsigned int flags;
> >>  	u16 queue_index;
> >> +	bool enabled;
> >> +	struct list_head next;
> >>  };
> >>  
> >>  static struct proto macvtap_proto = {
> >> @@ -85,14 +87,37 @@ static const struct proto_ops macvtap_socket_ops;
> >>   */
> >>  static DEFINE_SPINLOCK(macvtap_lock);
> >>  
> >> -static int macvtap_set_queue(struct net_device *dev, struct file *file,
> >> +static int macvtap_enable_queue(struct net_device *dev, struct file *file,
> >>  				struct macvtap_queue *q)
> >>  {
> >>  	struct macvlan_dev *vlan = netdev_priv(dev);
> >> +	int err = -EINVAL;
> >> +
> >> +	spin_lock(&macvtap_lock);
> >> +
> >> +	if (q->enabled)
> >> +		goto out;
> >> +
> >> +	err = 0;
> >> +	rcu_assign_pointer(vlan->taps[vlan->numvtaps], q);
> >> +	q->queue_index = vlan->numvtaps;
> >> +	q->enabled = true;
> >> +
> >> +	vlan->numvtaps++;
> >> +	vlan->numdisabled--;
> >> +out:
> >> +	spin_unlock(&macvtap_lock);
> >> +	return err;
> >> +}
> >> +
> >> +static int macvtap_set_queue(struct net_device *dev, struct file *file,
> >> +			     struct macvtap_queue *q)
> >> +{
> >> +	struct macvlan_dev *vlan = netdev_priv(dev);
> >>  	int err = -EBUSY;
> >>  
> >>  	spin_lock(&macvtap_lock);
> >> -	if (vlan->numvtaps == MAX_MACVTAP_QUEUES)
> >> +	if (vlan->numvtaps + vlan->numdisabled == MAX_MACVTAP_QUEUES)
> >>  		goto out;
> >>  
> >>  	err = 0;
> >> @@ -102,7 +127,9 @@ static int macvtap_set_queue(struct net_device *dev, struct file *file,
> >>  
> >>  	q->file = file;
> >>  	q->queue_index = vlan->numvtaps;
> >> +	q->enabled = true;
> >>  	file->private_data = q;
> >> +	list_add_tail(&q->next, &vlan->tap_link);
> >>  
> >>  	vlan->numvtaps++;
> >>  
> >> @@ -111,6 +138,38 @@ out:
> >>  	return err;
> >>  }
> >>  
> >> +static int macvtap_disable_queue(struct macvtap_queue *q)
> >> +{
> >> +	struct macvlan_dev *vlan;
> >> +	struct macvtap_queue *nq;
> >> +	int err = 0;
> >> +
> >> +	spin_lock(&macvtap_lock);
> >> +	vlan = rcu_dereference_protected(q->vlan,
> >> +					 lockdep_is_held(&macvtap_lock));
> >> +
> >> +	if (!q->enabled) {
> >> +		err = -EINVAL;
> >> +		goto out;
> >> +	}
> >> +
> >> +	if (vlan) {
> >> +		int index = q->queue_index;
> >> +		nq = rcu_dereference_protected(vlan->taps[vlan->numvtaps - 1],
> >> +					       lockdep_is_held(&macvtap_lock));
> >> +		nq->queue_index = index;
> >> +
> >> +		rcu_assign_pointer(vlan->taps[index], nq);
> >> +		RCU_INIT_POINTER(vlan->taps[vlan->numvtaps - 1], NULL);
> >> +		q->enabled = false;
> >> +
> >> +		vlan->numvtaps--;
> >> +		vlan->numdisabled++;
> >> +	}
> >> +out:
> >> +	spin_unlock(&macvtap_lock);
> >> +	return err;
> >> +}
> >>  /*
> >>   * The file owning the queue got closed, give up both
> >>   * the reference that the files holds as well as the
> >> @@ -128,18 +187,24 @@ static void macvtap_put_queue(struct macvtap_queue *q)
> >>  	vlan = rcu_dereference_protected(q->vlan,
> >>  					 lockdep_is_held(&macvtap_lock));
> >>  	if (vlan) {
> >> +		int numvtaps = vlan->numvtaps;
> >>  		int index = q->queue_index;
> >> -		BUG_ON(index >= vlan->numvtaps);
> >>  
> >> -		nq = rcu_dereference_protected(vlan->taps[vlan->numvtaps - 1],
> >> -					       lockdep_is_held(&macvtap_lock));
> >> -		rcu_assign_pointer(vlan->taps[index], nq);
> >> -		nq->queue_index = index;
> >> +		if (q->enabled) {
> >> +			BUG_ON(index >= vlan->numvtaps);
> >> +			nq = rcu_dereference_protected(vlan->taps[numvtaps - 1],
> >> +						lockdep_is_held(&macvtap_lock));
> >> +			rcu_assign_pointer(vlan->taps[index], nq);
> > Do we really need these tricks?
> > Can't we call macvtap_disable_queue and then only handle disable queues
> > here?
> 
> We could, will do it.
> >
> >> +			nq->queue_index = index;
> >> +
> >> +			RCU_INIT_POINTER(vlan->taps[numvtaps - 1], NULL);
> >> +			vlan->numvtaps--;
> >> +		} else
> >> +			vlan->numdisabled--;
> >>  
> >> -		RCU_INIT_POINTER(vlan->taps[vlan->numvtaps - 1], NULL);
> >>  		RCU_INIT_POINTER(q->vlan, NULL);
> >>  		sock_put(&q->sk);
> >> -		--vlan->numvtaps;
> >> +		list_del_init(&q->next);
> >>  	}
> >>  
> >>  	spin_unlock(&macvtap_lock);
> >> @@ -160,6 +225,10 @@ static struct macvtap_queue *macvtap_get_queue(struct net_device *dev,
> >>  {
> >>  	struct macvlan_dev *vlan = netdev_priv(dev);
> >>  	struct macvtap_queue *tap = NULL;
> >> +	/* numvtaps here is just a hint, even if the number of active taps were
> >> +	 * reduced in the same sime,
> > typo
> 
> Will correct it.
> >
> >> since we are protected by rcu read lock,
> >> +	 * we're safe here.
> > I don't really understand what is this comment trying to say.
> > What is protected? What is safe? safe time as what?
> 
> Will make it clear as:
> 
> "numvtaps is just a hint, even if the number of active taps were reduced
> in the same tap, since the tap pointers were protected by rcu read lock,
> they are safe even if some of them were being removed"

I think I see.  I think what you really meant is two things:
- near macvtap_forward:
 /* called under rcu read lock */
- and here: 
/*
 * Access to taps array is protected by rcu, but access to numvtaps
 * isn't. Below we use it to lookup a queue, but treat it as a hint
 * and validate that the result isn't NULL - in case we are
 * racing against queue removal.
 */

> >> +	 */
> >>  	int numvtaps = ACCESS_ONCE(vlan->numvtaps);
> >>  	__u32 rxq;
> >>  
> >> @@ -196,20 +265,25 @@ out:
> >>  static void macvtap_del_queues(struct net_device *dev)
> >>  {
> >>  	struct macvlan_dev *vlan = netdev_priv(dev);
> >> -	struct macvtap_queue *q, *qlist[MAX_MACVTAP_QUEUES];
> >> +	struct macvtap_queue *q, *tmp, *qlist[MAX_MACVTAP_QUEUES];
> >>  	int i, j = 0;
> >>  
> >>  	spin_lock(&macvtap_lock);
> >> -	for (i = 0; i < vlan->numvtaps; i++) {
> >> -		q = rcu_dereference_protected(vlan->taps[i],
> >> -					      lockdep_is_held(&macvtap_lock));
> >> -		BUG_ON(q == NULL);
> >> +	list_for_each_entry_safe(q, tmp, &vlan->tap_link, next) {
> >> +		list_del_init(&q->next);
> >>  		qlist[j++] = q;
> >> -		RCU_INIT_POINTER(vlan->taps[i], NULL);
> >>  		RCU_INIT_POINTER(q->vlan, NULL);
> >> +		if (q->enabled)
> >> +			vlan->numvtaps--;
> >> +		else
> >> +			vlan->numdisabled--;
> >>  	}
> >> +	for (i = 0; i < vlan->numvtaps; i++)
> >> +		RCU_INIT_POINTER(vlan->taps[i], NULL);
> >> +	BUG_ON(vlan->numvtaps + vlan->numdisabled != 0);
> > Better two BUG_ON checks - they are both 0 right?
> > And no need for != 0.
> 
> Sure.
> >
> >>  	/* guarantee that any future macvtap_set_queue will fail */
> >>  	vlan->numvtaps = MAX_MACVTAP_QUEUES;
> >> +	vlan->numdisabled = 0;
> > What's this doing? We know it's 0 ...
> 
> Yes, will remove it.
> >>  	spin_unlock(&macvtap_lock);
> >>  
> >>  	synchronize_rcu();
> >> @@ -298,6 +372,9 @@ static int macvtap_newlink(struct net *src_net,
> >>  			   struct nlattr *tb[],
> >>  			   struct nlattr *data[])
> >>  {
> >> +	struct macvlan_dev *vlan = netdev_priv(dev);
> >> +	INIT_LIST_HEAD(&vlan->tap_link);
> >> +
> >>  	/* Don't put anything that may fail after macvlan_common_newlink
> >>  	 * because we can't undo what it does.
> >>  	 */
> >> @@ -926,6 +1003,27 @@ static int macvtap_set_iff(struct file *file, struct ifreq __user *ifr_u)
> >>  	return 0;
> >>  }
> >>  
> >> +static int macvtap_ioctl_set_queue(struct file *file, unsigned int flags)
> >> +{
> >> +	struct macvtap_queue *q = file->private_data;
> >> +	struct macvlan_dev *vlan;
> >> +	int ret = -EINVAL;
> >> +
> >> +	vlan = macvtap_get_vlan(q);
> >> +	if (!vlan)
> >> +		goto done;
> > Can be just return -EINVAL, and you won't need to initialize
> > above.
> 
> Ok.
> >
> >> +
> >> +	if (flags & IFF_ATTACH_QUEUE)
> >> +		ret = macvtap_enable_queue(vlan->dev, file, q);
> >> +	else if (flags & IFF_DETACH_QUEUE)
> >> +		ret = macvtap_disable_queue(q);
> >> +
> >> +	macvtap_put_vlan(vlan);
> >> +
> >> +done:
> >> +	return ret;
> >> +}
> >> +
> >>  /*
> >>   * provide compatibility with generic tun/tap interface
> >>   */
> >> @@ -958,6 +1056,11 @@ static long macvtap_ioctl(struct file *file, unsigned int cmd,
> >>  		macvtap_put_vlan(vlan);
> >>  		return ret;
> >>  
> >> +	case TUNSETQUEUE:
> >> +		if (get_user(u, &ifr->ifr_flags))
> >> +			return -EFAULT;
> >> +		return macvtap_ioctl_set_queue(file, u);
> >> +
> >>  	case TUNGETFEATURES:
> >>  		if (put_user(IFF_TAP | IFF_NO_PI | IFF_VNET_HDR, up))
> >>  			return -EFAULT;
> >> diff --git a/include/linux/if_macvlan.h b/include/linux/if_macvlan.h
> >> index 62d8bda..87b9091 100644
> >> --- a/include/linux/if_macvlan.h
> >> +++ b/include/linux/if_macvlan.h
> >> @@ -69,8 +69,12 @@ struct macvlan_dev {
> >>  	u16			flags;
> >>  	int (*receive)(struct sk_buff *skb);
> >>  	int (*forward)(struct net_device *dev, struct sk_buff *skb);
> >> +	/* This array tracks active taps. */
> >>  	struct macvtap_queue	*taps[MAX_MACVTAP_QUEUES];
> >> +	/* This list tracks all taps (both enabled and disabled) */
> >> +	struct list_head	tap_link;
> > Bad name, e.g. queue_list would be better.
> 
> Ok.
> >>  	int			numvtaps;
> >> +	int			numdisabled;
> > So numvtaps is number of entries in the taps array.
> > But to get items on list, you must add numdisabled and
> > numvtaps, that's confusing.
> > Instead of numdisabled, let's add a counter for
> > number of entries in the tap_link list.
> >
> > How about numqueues?
> 
> This sounds better.
> >>  	int			minor;
> >>  };
> >>  
> >> -- 
> >> 1.7.1