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

From: Jason Wang <[email-masked]>
Date: Fri, 24 May 2013 13:33:17 +0800

On 05/23/2013 07:41 PM, Michael S. Tsirkin wrote:
> 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?

Two refcnts were held, one is the socket itself (and we can safely put
it here), another is held by device when doing macvtap_set_queue(), it
was released by another sock_put() after synchronize_rcu().
>
>>  		--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?

Same and cleaner, thanks.
>
>> @@ -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