Subject: Re: [net-next RFC 5/8] macvtap: eliminate linear search

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 23 May 2013 14:41:09 +0300

On Thu, May 23, 2013 at 11:12:30AM +0800, Jason Wang wrote:
> Linear search were used in both get_slot() and macvtap_get_queue(), this is
> because:
> 
> - macvtap didn't reshuffle the array of taps when create or destroy a queue, so
>   when adding a new queue, macvtap must do linear search to find a location for
>   the new queue. This will also complicate the TUNSETQUEUE implementation for
>   multiqueue API.
> - the queue itself didn't track the queue index, so the we must do a linear
>   search in the array to find the location of a existed queue.
> 
> The solution is straightforward: reshuffle the array and introduce a queue_index
> to macvtap_queue.
> 
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/net/macvtap.c |   63 +++++++++++++++---------------------------------
>  1 files changed, 20 insertions(+), 43 deletions(-)
> 
> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> index a36e49e..5fd341c 100644
> --- a/drivers/net/macvtap.c
> +++ b/drivers/net/macvtap.c
> @@ -44,6 +44,7 @@ struct macvtap_queue {
>  	struct macvlan_dev __rcu *vlan;
>  	struct file *file;
>  	unsigned int flags;
> +	u16 queue_index;
>  };
>  
>  static struct proto macvtap_proto = {
> @@ -84,30 +85,10 @@ static const struct proto_ops macvtap_socket_ops;
>   */
>  static DEFINE_SPINLOCK(macvtap_lock);
>  
> -/*
> - * get_slot: return a [unused/occupied] slot in vlan->taps[]:
> - *	- if 'q' is NULL, return the first empty slot;
> - *	- otherwise, return the slot this pointer occupies.
> - */
> -static int get_slot(struct macvlan_dev *vlan, struct macvtap_queue *q)
> -{
> -	int i;
> -
> -	for (i = 0; i < MAX_MACVTAP_QUEUES; i++) {
> -		if (rcu_dereference_protected(vlan->taps[i],
> -					      lockdep_is_held(&macvtap_lock)) == q)
> -			return i;
> -	}
> -
> -	/* Should never happen */
> -	BUG_ON(1);
> -}
> -
>  static int macvtap_set_queue(struct net_device *dev, struct file *file,
>  				struct macvtap_queue *q)
>  {
>  	struct macvlan_dev *vlan = netdev_priv(dev);
> -	int index;
>  	int err = -EBUSY;
>  
>  	spin_lock(&macvtap_lock);
> @@ -115,12 +96,12 @@ static int macvtap_set_queue(struct net_device *dev, struct file *file,
>  		goto out;
>  
>  	err = 0;
> -	index = get_slot(vlan, NULL);
>  	rcu_assign_pointer(q->vlan, vlan);
> -	rcu_assign_pointer(vlan->taps[index], q);
> +	rcu_assign_pointer(vlan->taps[vlan->numvtaps], q);
>  	sock_hold(&q->sk);
>  
>  	q->file = file;
> +	q->queue_index = vlan->numvtaps;
>  	file->private_data = q;
>  
>  	vlan->numvtaps++;
> @@ -140,16 +121,23 @@ 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 index = get_slot(vlan, q);
> +		int index = q->queue_index;
> +		BUG_ON(index >= vlan->numvtaps);
>  
> -		RCU_INIT_POINTER(vlan->taps[index], NULL);
> +		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;
> +
>  		sock_put(&q->sk);

Hmm this looks like a bug BTW: sock_put should be after
no one uses this tap, so must be after synchronize_rcu.
No?

>  		--vlan->numvtaps;
>  	}

So:
		nq = rcu_dereference_protected(vlan->taps[vlan->numvtaps - 1],
					       lockdep_is_held(&macvtap_lock));
		rcu_assign_pointer(vlan->taps[index], nq);

same thing?

> @@ -182,8 +170,7 @@ static struct macvtap_queue *macvtap_get_queue(struct net_device *dev,
>  	rxq = skb_get_rxhash(skb);
>  	if (rxq) {
>  		tap = rcu_dereference(vlan->taps[rxq % numvtaps]);
> -		if (tap)
> -			goto out;
> +		goto out;
>  	}
>  
>  	if (likely(skb_rx_queue_recorded(skb))) {
> @@ -193,17 +180,10 @@ static struct macvtap_queue *macvtap_get_queue(struct net_device *dev,
>  			rxq -= numvtaps;
>  
>  		tap = rcu_dereference(vlan->taps[rxq]);
> -		if (tap)
> -			goto out;
> -	}
> -
> -	/* Everything failed - find first available queue */
> -	for (rxq = 0; rxq < MAX_MACVTAP_QUEUES; rxq++) {
> -		tap = rcu_dereference(vlan->taps[rxq]);
> -		if (tap)
> -			break;
> +		goto out;
>  	}
>  
> +	tap = rcu_dereference(vlan->taps[0]);
>  out:
>  	return tap;
>  }
> @@ -221,17 +201,14 @@ 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 < MAX_MACVTAP_QUEUES && vlan->numvtaps; i++) {
> +	for (i = 0; i < vlan->numvtaps; i++) {
>  		q = rcu_dereference_protected(vlan->taps[i],
>  					      lockdep_is_held(&macvtap_lock));
> -		if (q) {
> -			qlist[j++] = q;
> -			RCU_INIT_POINTER(vlan->taps[i], NULL);
> -			RCU_INIT_POINTER(q->vlan, NULL);
> -			vlan->numvtaps--;
> -		}
> +		BUG_ON(q == NULL);
> +		qlist[j++] = q;
> +		RCU_INIT_POINTER(vlan->taps[i], NULL);
> +		RCU_INIT_POINTER(q->vlan, NULL);
>  	}
> -	BUG_ON(vlan->numvtaps != 0);
>  	/* guarantee that any future macvtap_set_queue will fail */
>  	vlan->numvtaps = MAX_MACVTAP_QUEUES;
>  	spin_unlock(&macvtap_lock);
> -- 
> 1.7.1