Subject: Re: [PATCH V6 3/3] tuntap: allow polling/writing/reading when detached

From: Jason Wang <[email-masked]>
Date: Thu, 17 Jan 2013 09:16:20 +0800

On 01/17/2013 01:03 AM, Michael S. Tsirkin wrote:
> On Wed, Jan 16, 2013 at 11:44:38PM +0800, Jason Wang wrote:
>> We forbid polling, writing and reading when the file were detached, this may
>> complex the user in several cases:
>>
>> - when guest pass some buffers to vhost/qemu and then disable some queues,
>>   host/qemu needs to do its own cleanup on those buffers which is complex
>>   sometimes. We can do this simply by allowing a user can still write to an
>>   disabled queue. Write to an disabled queue will cause the packet pass to the
>>   kernel and read will get nothing.
>> - align the polling behavior with macvtap which never fails when the queue is
>>   created. This can simplify the polling errors handling of its user (e.g vhost)
>>
>> In order to achieve this, tfile->tun were not assign to NULL when detached. And
>> tfile->tun were converted to be RCU protected in order to let the data path can
>> check whether the file is deated in a lockless manner. This will be used to
>> prevent the flow caches from being updated for a detached queue.
>>
>> Signed-off-by: Jason Wang <[email-masked]>
>> ---
>>  drivers/net/tun.c |   45 ++++++++++++++++++++++++++-------------------
>>  1 files changed, 26 insertions(+), 19 deletions(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index c81680d..ec539a9 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -139,7 +139,7 @@ struct tun_file {
>>  	unsigned int flags;
>>  	u16 queue_index;
>>  	struct list_head next;
>> -	struct tun_struct *detached;
>> +	struct tun_struct __rcu *detached;
>>  };
>>  
>>  struct tun_flow_entry {
>> @@ -295,11 +295,12 @@ static void tun_flow_cleanup(unsigned long data)
>>  }
>>  
>>  static void tun_flow_update(struct tun_struct *tun, u32 rxhash,
>> -			    u16 queue_index)
>> +			    struct tun_file *tfile)
>>  {
>>  	struct hlist_head *head;
>>  	struct tun_flow_entry *e;
>>  	unsigned long delay = tun->ageing_time;
>> +	u16 queue_index = tfile->queue_index;
>>  
>>  	if (!rxhash)
>>  		return;
>> @@ -308,7 +309,7 @@ static void tun_flow_update(struct tun_struct *tun, u32 rxhash,
>>  
>>  	rcu_read_lock();
>>  
>> -	if (tun->numqueues == 1)
>> +	if (tun->numqueues == 1 || rcu_dereference(tfile->detached))
>>  		goto unlock;
>>  
>>  	e = tun_flow_find(head, rxhash);
> Sorry, still an issue with this one.

No problem, thanks for the checking.
>
>                 u16 index = tfile->queue_index;
>                 BUG_ON(index >= tun->numqueues);
>                 dev = tun->dev;
>
>                 rcu_assign_pointer(tun->tfiles[index],
>                                    tun->tfiles[tun->numqueues - 1]);
>                 rcu_assign_pointer(tfile->tun, NULL);
>                 ntfile = rtnl_dereference(tun->tfiles[index]);
>                 ntfile->queue_index = index;
>
>                 --tun->numqueues;
>                 if (clean)
>                         sock_put(&tfile->sk);
>                 else
>                         tun_disable_queue(tun, tfile);
>
> You should first disable queue then synchronize network
> only then play with tfiles array.
> As it is you might have removed file from array but
> did not set detached flag yet, so queue_index
> above is stable.

I think the code is ok here. With this patch, before synchronize_net(), 
the only thing we do for the tfile that will be detached is to set the
tfile->detached (tun_disable_queue), and the queue_index is kept
unchanged. So if the data path don't see the new value of detached, it
still can treat the tfile is undetached and do the sending and receiving
as usual. We only do the cleanup after the synchronization which all
reader are guaranteed to see the new detached value.

For the tfile that will be moved to the new place, some (should be very
little) OOO will occur which I think is acceptable and can be optimized
in the future.
>
> On enable, clear detached last thing.
>
>> @@ -384,16 +385,16 @@ static void tun_set_real_num_queues(struct tun_struct *tun)
>>  
>>  static void tun_disable_queue(struct tun_struct *tun, struct tun_file *tfile)
>>  {
>> -	tfile->detached = tun;
>> +	rcu_assign_pointer(tfile->detached, tun);
>>  	list_add_tail(&tfile->next, &tun->disabled);
>>  	++tun->numdisabled;
>>  }
>>  
>>  static struct tun_struct *tun_enable_queue(struct tun_file *tfile)
>>  {
>> -	struct tun_struct *tun = tfile->detached;
>> +	struct tun_struct *tun = rtnl_dereference(tfile->detached);
>>  
>> -	tfile->detached = NULL;
>> +	rcu_assign_pointer(tfile->detached, NULL);
>>  	list_del_init(&tfile->next);
>>  	--tun->numdisabled;
>>  	return tun;
>> @@ -402,26 +403,27 @@ static struct tun_struct *tun_enable_queue(struct tun_file *tfile)
>>  static void __tun_detach(struct tun_file *tfile, bool clean)
>>  {
>>  	struct tun_file *ntfile;
>> -	struct tun_struct *tun;
>> +	struct tun_struct *tun, *detached;
>>  	struct net_device *dev;
>>  
>>  	tun = rtnl_dereference(tfile->tun);
>> +	detached = rtnl_dereference(tfile->detached);
>>  
>> -	if (tun) {
>> +	if (tun && !detached) {
>>  		u16 index = tfile->queue_index;
>>  		BUG_ON(index >= tun->numqueues);
>>  		dev = tun->dev;
>>  
>>  		rcu_assign_pointer(tun->tfiles[index],
>>  				   tun->tfiles[tun->numqueues - 1]);
>> -		rcu_assign_pointer(tfile->tun, NULL);
>>  		ntfile = rtnl_dereference(tun->tfiles[index]);
>>  		ntfile->queue_index = index;
>>  
>>  		--tun->numqueues;
>> -		if (clean)
>> +		if (clean) {
>> +			rcu_assign_pointer(tfile->tun, NULL);
>>  			sock_put(&tfile->sk);
>> -		else
>> +		} else
>>  			tun_disable_queue(tun, tfile);
>>  
>>  		synchronize_net();
>> @@ -429,7 +431,7 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
>>  		/* Drop read queue */
>>  		skb_queue_purge(&tfile->sk.sk_receive_queue);
>>  		tun_set_real_num_queues(tun);
>> -	} else if (tfile->detached && clean) {
>> +	} else if (detached && clean) {
>>  		tun = tun_enable_queue(tfile);
>>  		sock_put(&tfile->sk);
>>  	}
>> @@ -466,6 +468,10 @@ static void tun_detach_all(struct net_device *dev)
>>  		rcu_assign_pointer(tfile->tun, NULL);
>>  		--tun->numqueues;
>>  	}
>> +	list_for_each_entry(tfile, &tun->disabled, next) {
>> +		wake_up_all(&tfile->wq.wait);
>> +		rcu_assign_pointer(tfile->tun, NULL);
>> +	}
>>  	BUG_ON(tun->numqueues != 0);
>>  
>>  	synchronize_net();
>> @@ -496,7 +502,7 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
>>  		goto out;
>>  
>>  	err = -EINVAL;
>> -	if (rtnl_dereference(tfile->tun))
>> +	if (rtnl_dereference(tfile->tun) && !rtnl_dereference(tfile->detached))
>>  		goto out;
>>  
>>  	err = -EBUSY;
>> @@ -504,7 +510,7 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
>>  		goto out;
>>  
>>  	err = -E2BIG;
>> -	if (!tfile->detached &&
>> +	if (!rtnl_dereference(tfile->detached) &&
>>  	    tun->numqueues + tun->numdisabled == MAX_TAP_QUEUES)
>>  		goto out;
>>  
>> @@ -521,7 +527,7 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
>>  	rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
>>  	tun->numqueues++;
>>  
>> -	if (tfile->detached)
>> +	if (rtnl_dereference(tfile->detached))
>>  		tun_enable_queue(tfile);
>>  	else
>>  		sock_hold(&tfile->sk);
>> @@ -1195,7 +1201,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>>  	tun->dev->stats.rx_packets++;
>>  	tun->dev->stats.rx_bytes += len;
>>  
>> -	tun_flow_update(tun, rxhash, tfile->queue_index);
>> +	tun_flow_update(tun, rxhash, tfile);
>>  	return total_len;
>>  }
>>  
>> @@ -1552,7 +1558,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>>  	struct net_device *dev;
>>  	int err;
>>  
>> -	if (tfile->detached)
>> +	if (rtnl_dereference(tfile->detached))
>>  		return -EINVAL;
>>  
>>  	dev = __dev_get_by_name(net, ifr->ifr_name);
>> @@ -1796,7 +1802,7 @@ static int tun_set_queue(struct file *file, struct ifreq *ifr)
>>  	rtnl_lock();
>>  
>>  	if (ifr->ifr_flags & IFF_ATTACH_QUEUE) {
>> -		tun = tfile->detached;
>> +		tun = rtnl_dereference(tfile->detached);
>>  		if (!tun) {
>>  			ret = -EINVAL;
>>  			goto unlock;
>> @@ -1807,7 +1813,8 @@ static int tun_set_queue(struct file *file, struct ifreq *ifr)
>>  		ret = tun_attach(tun, file);
>>  	} else if (ifr->ifr_flags & IFF_DETACH_QUEUE) {
>>  		tun = rtnl_dereference(tfile->tun);
>> -		if (!tun || !(tun->flags & TUN_TAP_MQ))
>> +		if (!tun || !(tun->flags & TUN_TAP_MQ) ||
>> +		    rtnl_dereference(tfile->detached))
>>  			ret = -EINVAL;
>>  		else
>>  			__tun_detach(tfile, false);
>> -- 
>> 1.7.1
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