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

From: Jason Wang <[email-masked]>
Date: Thu, 24 Jan 2013 18:12:44 +0800

On 01/17/2013 09:16 AM, Jason Wang wrote:
> 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.

Having a thought about this patch, looks like it's suboptimal since:

- If we can make sure no packets were sent to the disabled queue and
stop the vhost thread during switching (then it can flush). There's no
need for this patch.
- Allowing writing/polling to a detached fd is strange and can hide the
bugs of userspace / guest driver.

So looks like we'd better drop this patch?
>> 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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