Subject: Re: [net-next rfc V2 7/8] macvtap: add TUNSETQUEUE ioctl

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 3 Jun 2013 14:09:24 +0300

On Mon, Jun 03, 2013 at 01:20:58PM +0800, Jason Wang wrote:
> On 06/02/2013 07:22 PM, Michael S. Tsirkin wrote:
> > On Fri, May 31, 2013 at 05:53:24PM +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 by split the taps array into three different areas:
> >>
> >> - [0, numvtaps) : enabled taps
> >> - [numvtaps, numvtaps + numdisabled) : disabled taps
> >> - [numvtaps + numdisabled, MAX_MAXVTAP_QUEUES) : unused slots
> >>
> >> When a tap were enabled and disabled, it was moved to another area.
> >>
> >> Signed-off-by: Jason Wang <[email-masked]>
> >
> > This seems a bit tricky. Can we just move the tap out of the
> > array? 
> 
> Certainly yes.
> > the only reason we have the array is for fast
> > lookup on xmit.
> > What's the reason to keep disabled queues there?
> 
> It saves us some space and make code simpler.
> > To be able to track all queues for cleanups, all we need is
> > a linked list of all queues (enabled and disabled).
> 
> Yes, but you need iterate both arrays and linked list which won't be
> simpler than keeping them in place.

No, my idea is to keep all taps in the list.

If you need all taps, walks the list.
If you need active taps, look them up in the array.

Reasonable?

> >> ---
> >>  drivers/net/macvtap.c      |  167 ++++++++++++++++++++++++++++++++++++++++----
> >>  include/linux/if_macvlan.h |    7 ++
> >>  2 files changed, 159 insertions(+), 15 deletions(-)
> >>
> >> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> >> index eac49cb..03b781c 100644
> >> --- a/drivers/net/macvtap.c
> >> +++ b/drivers/net/macvtap.c
> >> @@ -85,32 +85,126 @@ 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 void macvtap_swap_slot(struct macvlan_dev *vlan, int a, int b)
> >> +{
> >> +	struct macvtap_queue *q1, *q2;
> >> +
> >> +	if (a == b)
> >> +		return;
> >> +
> >> +	q1 = rcu_dereference_protected(vlan->taps[a],
> >> +				       lockdep_is_held(&macvtap_lock));
> >> +	q2 = rcu_dereference_protected(vlan->taps[b],
> >> +				       lockdep_is_held(&macvtap_lock));
> >> +
> >> +	BUG_ON(q1 == NULL || q2 == NULL);
> >> +
> >> +	rcu_assign_pointer(vlan->taps[a], q2);
> >> +	rcu_assign_pointer(vlan->taps[b], q1);
> >> +
> >> +	q1->queue_index = b;
> >> +	q2->queue_index = a;
> >> +}
> >> +
> >> +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;
> >> +	int total;
> >> +
> >> +	spin_lock(&macvtap_lock);
> >> +	total = vlan->numvtaps + vlan->numdisabled;
> >> +
> >> +	if (q->queue_index < vlan->numvtaps)
> >> +		goto out;
> >> +
> >> +	err = 0;
> >> +
> >> +	BUG_ON(q->queue_index >= total);
> >> +	macvtap_swap_slot(vlan, q->queue_index, vlan->numvtaps);
> >> +
> >> +	/* Make sure the pointers were seen before indices. */
> >> +	wmb();
> > Which indices?  We only care about numvtaps right?
> > So let's just say so.
> 
> ok
> >
> > Why is this wmb and not smp_wmb()?
> 
> will correct it.
> >
> >> +
> >> +	vlan->numdisabled--;
> >> +	vlan->numvtaps++;
> >> +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;
> >> +	int total;
> >>  
> >>  	spin_lock(&macvtap_lock);
> >> -	if (vlan->numvtaps == MAX_MACVTAP_QUEUES)
> >> +
> >> +	total = vlan->numvtaps + vlan->numdisabled;
> >> +	if (total == MAX_MACVTAP_QUEUES)
> >>  		goto out;
> >>  
> >>  	err = 0;
> >> +
> >>  	rcu_assign_pointer(q->vlan, vlan);
> >> -	rcu_assign_pointer(vlan->taps[vlan->numvtaps], q);
> >> +	rcu_assign_pointer(vlan->taps[total], q);
> >>  	sock_hold(&q->sk);
> >>  
> >>  	q->file = file;
> >> -	q->queue_index = vlan->numvtaps;
> >> +	q->queue_index = total;
> >>  	file->private_data = q;
> >> +	if (vlan->numdisabled)
> >> +		macvtap_swap_slot(vlan, vlan->numvtaps, total);
> >>  
> >> -	vlan->numvtaps++;
> >> +	/* Make sure the pointers were seen before indices. */
> >> +	wmb();
> >>  
> >> +	vlan->numvtaps++;
> >>  out:
> >>  	spin_unlock(&macvtap_lock);
> >>  	return err;
> >>  }
> >>  
> >> +static int macvtap_disable_queue(struct macvtap_queue *q)
> >> +{
> >> +	struct macvlan_dev *vlan;
> >> +	int err = -EINVAL;
> >> +
> >> +	spin_lock(&macvtap_lock);
> >> +	vlan = rcu_dereference_protected(q->vlan,
> >> +					 lockdep_is_held(&macvtap_lock));
> >> +
> >> +	if (vlan) {
> >> +		int total = vlan->numvtaps + vlan->numdisabled;
> >> +		int index = q->queue_index;
> >> +
> >> +		BUG_ON(q->queue_index >= total);
> >> +		if (q->queue_index >= vlan->numvtaps)
> >> +			goto out;
> >> +
> >> +		err = 0;
> >> +		macvtap_swap_slot(vlan, index, total - 1);
> >> +		if (vlan->numdisabled)
> >> +			/* If there's disabled taps, the above swap will cause
> >> +			 * a disabled tap to be moved to enabled area. So
> >> +			 * another swap is needed to keep the right order.
> >> +			 */
> >> +			macvtap_swap_slot(vlan, index, vlan->numvtaps - 1);
> >> +
> >> +		/* make sure the pointers were seen before indices */
> >> +		wmb();
> > Hmm this looks questionable. The code near rmb first
> > checks numvtaps then dereferences the queue.
> > So it might see a new queue but old value of numvtaps.
> 
> Right, barriers here were just best effort to reduce the possibility of
> wrong queue selection when changing the number of queues.

If this is an optimization, I'd say benchmark it and
see if it helps performance.

I don't expect it to have any effect really.
In fact, the re-ordering of queues that this patch does
is likely to cause packet reorering and hurt performance
more.

I'm guessing the only thing we need for correctness
is ACCESS_ONCE on numvtaps?

> >
> >> +
> >> +		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
> >> @@ -121,25 +215,38 @@ out:
> >>   */
> >>  static void macvtap_put_queue(struct macvtap_queue *q)
> >>  {
> >> -	struct macvtap_queue *nq;
> >>  	struct macvlan_dev *vlan;
> >>  
> >>  	spin_lock(&macvtap_lock);
> >>  	vlan = rcu_dereference_protected(q->vlan,
> >>  					 lockdep_is_held(&macvtap_lock));
> >> +
> >>  	if (vlan) {
> >> +		int total = vlan->numvtaps + vlan->numdisabled;
> >>  		int index = q->queue_index;
> >> -		BUG_ON(index >= vlan->numvtaps);
> >> +		bool disabled = q->queue_index >= vlan->numvtaps;
> >> +
> >> +		BUG_ON(q->queue_index >= total);
> >> +		macvtap_swap_slot(vlan, index, total - 1);
> >> +		if (!disabled && vlan->numdisabled)
> >> +			/* If there's disabled taps, the above swap will cause
> >> +			 * a disabled tap to be moved to enabled area. So
> >> +			 * another swap is needed to keep the right order.
> >> +			 */
> >> +			macvtap_swap_slot(vlan, index, vlan->numvtaps - 1);
> >> +
> >> +		RCU_INIT_POINTER(vlan->taps[total - 1], NULL);
> >> +		RCU_INIT_POINTER(q->vlan, NULL);
> >> +		sock_put(&q->sk);
> >>  
> >> -		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;
> >> +		/* Make sure the pointers were seen before indices */
> > Here it's one pointer, right?
> 
> Right.
> >
> >> +		wmb();
> > Same issue as above, looks even more worrying
> > as queue is freed here.
> 
> The read side were protected by rcu_read_lock(), so no worries here.

Okay so basically numvtaps is just a hint,
it can be wrong and nothing too bad happens?

OK, but let's document this.

> >
> >>  
> >> -		RCU_INIT_POINTER(q->vlan, NULL);
> >> +		if (disabled)
> >> +			vlan->numdisabled--;
> >> +		else
> >> +			vlan->numvtaps--;
> >>  
> >> -		sock_put(&q->sk);
> >> -		--vlan->numvtaps;
> >>  	}
> >>  
> >>  	spin_unlock(&macvtap_lock);
> >> @@ -166,6 +273,9 @@ static struct macvtap_queue *macvtap_get_queue(struct net_device *dev,
> >>  	if (!numvtaps)
> >>  		goto out;
> >>  
> >> +	/* Check taps after numvtaps were exposed. */
> >> +	rmb();
> >> +
> > Except this doesn't seem to handle case where taps are going away ...
> 
> We're protected by rcu read lock here so even though it choose the queue
> which is going to be destroyed temporarily, the socket won't be freed
> before the packets were queued in the socket.
> >>  	/* Check if we can use flow to select a queue */
> >>  	rxq = skb_get_rxhash(skb);
> >>  	if (rxq) {
> >> @@ -201,7 +311,7 @@ static void macvtap_del_queues(struct net_device *dev)
> >>  
> >>  	/* macvtap_put_queue can free some slots, so go through all slots */
> >>  	spin_lock(&macvtap_lock);
> >> -	for (i = 0; i < vlan->numvtaps; i++) {
> >> +	for (i = 0; i < vlan->numvtaps + vlan->numdisabled; i++) {
> >>  		q = rcu_dereference_protected(vlan->taps[i],
> >>  					      lockdep_is_held(&macvtap_lock));
> >>  		BUG_ON(q == NULL);
> >> @@ -211,6 +321,7 @@ static void macvtap_del_queues(struct net_device *dev)
> >>  	}
> >>  	/* guarantee that any future macvtap_set_queue will fail */
> >>  	vlan->numvtaps = MAX_MACVTAP_QUEUES;
> >> +	vlan->numdisabled = 0;
> >>  	spin_unlock(&macvtap_lock);
> >>  
> >>  	synchronize_rcu();
> >> @@ -927,6 +1038,27 @@ 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;
> >> +
> >> +	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
> >>   */
> >> @@ -959,6 +1091,11 @@ 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..d528f38 100644
> >> --- a/include/linux/if_macvlan.h
> >> +++ b/include/linux/if_macvlan.h
> >> @@ -69,8 +69,15 @@ struct macvlan_dev {
> >>  	u16			flags;
> >>  	int (*receive)(struct sk_buff *skb);
> >>  	int (*forward)(struct net_device *dev, struct sk_buff *skb);
> >> +	/* This array tracks all taps (include disabled ones) and will be
> >> +	 * reshuffled to keep the following order:
> >> +	 * [0, numvtaps) : enabled taps,
> >> +	 * [numvtaps, numvtaps + numdisabled) : disabled taps,
> >> +	 * [numvtaps + numdisabled, MAX_MACVTAP_QUEUES) : unused slots
> >> +	 */
> >>  	struct macvtap_queue	*taps[MAX_MACVTAP_QUEUES];
> >>  	int			numvtaps;
> >> +	int			numdisabled;
> >>  	int			minor;
> >>  };
> >>  
> >> -- 
> >> 1.7.1
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