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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 24 Jan 2013 12:37:25 +0200

On Thu, Jan 24, 2013 at 06:12:44PM +0800, Jason Wang wrote:
> 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.

This assumes synchronization in userspace/vhost, this will make
datapath slower without real need.

> - Allowing writing/polling to a detached fd

It's not a detached fd - it's attached to tun.
We just disabled receiving packets on it.

> is strange and can hide the
> bugs of userspace / guest driver.

That's a good thing, you don't want a fragile interface.

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

I actually think it's the right approach.
And since you clarified I think the patch is allright.

> >> 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
> >> --
> >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> >> the body of a message to [email-masked]
> >> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >> Please read the FAQ at  http://www.tux.org/lkml/
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> > the body of a message to [email-masked]
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at  http://www.tux.org/lkml/