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

From: Jason Wang <[email-masked]>
Date: Fri, 24 May 2013 14:19:08 +0800

On 05/23/2013 07:52 PM, Michael S. Tsirkin wrote:
> On Thu, May 23, 2013 at 11:12:32AM +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 just introduce a pointer detached which was set when the queue
>> were detached. Only the queue with NULL set to this pointer can be used to
>> forward packets.
>>
>> Signed-off-by: Jason Wang <[email-masked]>
> Isn't this a bit too tricky?

Well, not tricky and it was just like what we did the reshuffling in
PATCH 5/8
> Why keep disabled queues on the array at all?

Both are ok, but this method need less data structures in macvlan_dev.
> Let's add a linked list of all queues for accounting.
> Use array for enabled queues only, for data path.

This would not be simpler than just do in place reshuffling.
>
>> ---
>>  drivers/net/macvtap.c      |  147 +++++++++++++++++++++++++++++++++++--------
>>  include/linux/if_macvlan.h |    7 ++
>>  2 files changed, 126 insertions(+), 28 deletions(-)
>>
>> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
>> index e3f9344..06b10a5 100644
>> --- a/drivers/net/macvtap.c
>> +++ b/drivers/net/macvtap.c
>> @@ -85,67 +85,131 @@ static const struct proto_ops macvtap_socket_ops;
>>   */
>>  static DEFINE_SPINLOCK(macvtap_lock);
>>  
>> +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], vlan->taps[b]);
>> +	rcu_assign_pointer(vlan->taps[b], q1);
>> +
>> +	q1->queue_index = b;
>> +	q2->queue_index = a;
>> +}
>> +
>>  static int macvtap_set_queue(struct net_device *dev, struct file *file,
>> -				struct macvtap_queue *q)
>> +			     struct macvtap_queue *q, bool enable)
>>  {
>>  	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 (!enable && total == MAX_MACVTAP_QUEUES)
>> +		goto out;
>> +
>> +	err = -EINVAL;
>> +	if (enable && q->queue_index < vlan->numvtaps)
>>  		goto out;
>>  
>>  	err = 0;
>> -	rcu_assign_pointer(q->vlan, vlan);
>> -	rcu_assign_pointer(vlan->taps[vlan->numvtaps], q);
>> -	sock_hold(&q->sk);
>>  
>> -	q->file = file;
>> -	q->queue_index = vlan->numvtaps;
>> -	file->private_data = q;
>> +	if (enable) {
>> +		BUG_ON(q->queue_index >= total);
>> +		macvtap_swap_slot(vlan, q->queue_index, vlan->numvtaps);
>> +		--vlan->numdisabled;
>> +	} else {
>> +		rcu_assign_pointer(q->vlan, vlan);
>> +		rcu_assign_pointer(vlan->taps[total], q);
>> +		sock_hold(&q->sk);
>> +
>> +		q->file = file;
>> +		q->queue_index = total;
>> +		file->private_data = q;
>> +		if (vlan->numdisabled)
>> +			macvtap_swap_slot(vlan, vlan->numvtaps, total);
>> +
>> +		/* Make sure the pointers were seen before numvtaps. */
>> +		smp_wmb();
>> +	}
>>  
>>  	vlan->numvtaps++;
>> -
>>  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
>> - * one from the macvlan_dev if that still exists.
>> + * The file owning the queue got closed or disabled.
>> + *
>> + * If closed give up both the reference that the files holds as well as the
>> + * one from the macvlan_dev if that still exists. If disabled, reshuffle the
>> + * array of taps.
>>   *
>>   * Using the spinlock makes sure that we don't get
>>   * to the queue again after destroying it.
>>   */
>> -static void macvtap_put_queue(struct macvtap_queue *q)
>> +static int macvtap_put_queue(struct macvtap_queue *q, bool clean)
> s/clean/release/
> and add a comment explaining what it is.

Ok.
>>  {
>> -	struct macvtap_queue *nq;
>>  	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(index >= vlan->numvtaps);
>> +		bool disabled = q->queue_index >= vlan->numvtaps;
>>  
>> -		rcu_assign_pointer(vlan->taps[index],
>> -				   vlan->taps[vlan->numvtaps - 1]);
>> -		RCU_INIT_POINTER(q->vlan, NULL);
>> -		nq = rcu_dereference_protected(vlan->taps[index],
>> -					       lockdep_is_held(&macvtap_lock));
>> -		nq->queue_index = index;
>> +		if (!clean) {
>> +			BUG_ON(q->queue_index >= total);
>> +			if (q->queue_index >= vlan->numvtaps)
>> +				goto out;
>> +		}
>>  
>> -		sock_put(&q->sk);
>> -		--vlan->numvtaps;
>> -	}
>> +		err = 0;
>> +		macvtap_swap_slot(vlan, index, total - 1);
>> +		if (!disabled && vlan->numdisabled)
> What's the logic here?

If there's at least one disabled taps, the above swap will move a
disabled tap into enabled area. So we need to do swap it with the last
enabled tap to keep the right order.
>
>> +			macvtap_swap_slot(vlan, index, vlan->numvtaps - 1);
> So these swaps do a lot of RCU assign, presumably we need
> synchronize_rcu as well?

There is a synchronize_rcu() below, and we do the cleanup -sock_put()
after that. And since we purge the socket during its destruction, no
need to care about the little possibility that a packet were sent to the
disabled or removed queue.
>
>> +
>> +		if (clean) {
>> +			RCU_INIT_POINTER(vlan->taps[total - 1], NULL);
>> +			RCU_INIT_POINTER(q->vlan, NULL);
>> +			sock_put(&q->sk);
>> +			if (disabled)
>> +				--vlan->numdisabled;
>> +			else
>> +				--vlan->numvtaps;
>> +		} else {
>> +			--vlan->numvtaps;
>> +			++vlan->numdisabled;
>> +		}
>> +
>> +	} else if (clean)
>> +		err = 0;
>>  
>> +out:
>>  	spin_unlock(&macvtap_lock);
>>  
>>  	synchronize_rcu();
>> -	sock_put(&q->sk);
>> +
>> +	if (clean)
>> +		sock_put(&q->sk);
>> +
>> +	return err;
>>  }
>>  
>>  /*
>> @@ -201,7 +265,7 @@ 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 +275,7 @@ 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();
>> @@ -384,7 +449,7 @@ static int macvtap_open(struct inode *inode, struct file *file)
>>  	if ((dev->features & NETIF_F_HIGHDMA) && (dev->features & NETIF_F_SG))
>>  		sock_set_flag(&q->sk, SOCK_ZEROCOPY);
>>  
>> -	err = macvtap_set_queue(dev, file, q);
>> +	err = macvtap_set_queue(dev, file, q, false);
>>  	if (err)
>>  		sock_put(&q->sk);
>>  
>> @@ -398,7 +463,7 @@ out:
>>  static int macvtap_release(struct inode *inode, struct file *file)
>>  {
>>  	struct macvtap_queue *q = file->private_data;
>> -	macvtap_put_queue(q);
>> +	macvtap_put_queue(q, true);
>>  	return 0;
>>  }
>>  
>> @@ -922,6 +987,27 @@ 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_set_queue(vlan->dev, file, q, true);
>> +	else if (flags & IFF_DETACH_QUEUE)
>> +		ret = macvtap_put_queue(q, false);
>> +
>> +	macvtap_put_vlan(vlan);
>> +
>> +done:
>> +	return ret;
>> +}
>> +
>>  /*
>>   * provide compatibility with generic tun/tap interface
>>   */
>> @@ -954,6 +1040,11 @@ 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 32e943a..f134afb 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