Date:	Fri, 12 Aug 2011 16:21:31 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	mst@...hat.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, davem@...emloft.net,
	krkumar2@...ibm.com, rusty@...tcorp.com.au, qemu-devel@...gnu.org,
	kvm@...r.kernel.org, mirq-linux@...e.qmqm.pl
Subject: Re: [net-next RFC PATCH 4/7] tuntap: multiqueue support

On Fri, Aug 12, 2011 at 09:55:20AM +0800, Jason Wang wrote:
> With the abstraction that each socket were a backend of a
> queue for userspace, this patch adds multiqueue support for
> tap device by allowing multiple sockets to be attached to a
> tap device. Then we could parallize the transmission by put
> them into different socket.
> 
> As queue related information were stored in private_data of
> file new, we could simply implement the multiqueue support
> by add an array of pointers to sockets were stored in the
> tap device. Then ioctls may be added to manipulate those
> pointers for adding or removing queues.
> 
> In order to let tx path lockless, NETIF_F_LLTX were used for
> multiqueue tap device. And RCU is used for doing
> synchronization between packet handling and system calls
> such as removing queues.
> 
> Currently, multiqueue support is limited for tap , but it's
> easy also enable it for tun if we find it was also helpful.

Question below about calls to tun_get_slot().

							Thanx, Paul

> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  drivers/net/tun.c |  376 ++++++++++++++++++++++++++++++++++-------------------
>  1 files changed, 243 insertions(+), 133 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 4cd292a..8bc6dff 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -108,6 +108,8 @@ struct tap_filter {
>  	unsigned char	addr[FLT_EXACT_COUNT][ETH_ALEN];
>  };
> 
> +#define MAX_TAP_QUEUES (NR_CPUS < 16 ? NR_CPUS : 16)
> +
>  struct tun_file {
>  	struct sock sk;
>  	struct socket socket;
> @@ -115,7 +117,7 @@ struct tun_file {
>  	int vnet_hdr_sz;
>  	struct tap_filter txflt;
>  	atomic_t count;
> -	struct tun_struct *tun;
> +	struct tun_struct __rcu *tun;
>  	struct net *net;
>  	struct fasync_struct *fasync;
>  	unsigned int flags;
> @@ -124,7 +126,8 @@ struct tun_file {
>  struct tun_sock;
> 
>  struct tun_struct {
> -	struct tun_file		*tfile;
> +	struct tun_file		*tfiles[MAX_TAP_QUEUES];
> +	unsigned int            numqueues;
>  	unsigned int 		flags;
>  	uid_t			owner;
>  	gid_t			group;
> @@ -139,80 +142,183 @@ struct tun_struct {
>  #endif
>  };
> 
> -static int tun_attach(struct tun_struct *tun, struct file *file)
> +static DEFINE_SPINLOCK(tun_lock);
> +
> +/*
> + * get_slot: return a [unused/occupied] slot in tun->tfiles[]:
> + *     - if 'f' is NULL, return the first empty slot;
> + *     - otherwise, return the slot this pointer occupies.
> + */
> +static int tun_get_slot(struct tun_struct *tun, struct tun_file *tfile)
>  {
> -	struct tun_file *tfile = file->private_data;
> -	int err;
> +	int i;
> 
> -	ASSERT_RTNL();
> +	for (i = 0; i < MAX_TAP_QUEUES; i++) {
> +		if (rcu_dereference(tun->tfiles[i]) == tfile)
> +			return i;
> +	}
> 
> -	netif_tx_lock_bh(tun->dev);
> +       /* Should never happen */
> +       BUG_ON(1);
> +}
> 
> -	err = -EINVAL;
> -	if (tfile->tun)
> -		goto out;
> +/*
> + * tun_get_queue(): calculate the queue index
> + *     - if skbs comes from mq nics, we can just borrow
> + *     - if not, calculate from the hash
> + */
> +static struct tun_file *tun_get_queue(struct net_device *dev,
> +                                     struct sk_buff *skb)
> +{
> +	struct tun_struct *tun = netdev_priv(dev);
> +	struct tun_file *tfile = NULL;
> +	int numqueues = tun->numqueues;
> +	__u32 rxq;
> 
> -	err = -EBUSY;
> -	if (tun->tfile)
> +	BUG_ON(!rcu_read_lock_held());
> +
> +	if (!numqueues)
>  		goto out;
> 
> -	err = 0;
> -	tfile->tun = tun;
> -	tun->tfile = tfile;
> -	netif_carrier_on(tun->dev);
> -	dev_hold(tun->dev);
> -	sock_hold(&tfile->sk);
> -	atomic_inc(&tfile->count);
> +	if (likely(skb_rx_queue_recorded(skb))) {
> +		rxq = skb_get_rx_queue(skb);
> +
> +		while (unlikely(rxq >= numqueues))
> +			rxq -= numqueues;
> +
> +		tfile = rcu_dereference(tun->tfiles[rxq]);
> +		if (tfile)
> +			goto out;
> +	}
> +
> +	/* Check if we can use flow to select a queue */
> +	rxq = skb_get_rxhash(skb);
> +	if (rxq) {
> +		tfile = rcu_dereference(tun->tfiles[rxq % numqueues]);
> +		if (tfile)
> +			goto out;
> +	}
> +
> +	/* Everything failed - find first available queue */
> +	for (rxq = 0; rxq < MAX_TAP_QUEUES; rxq++) {
> +		tfile = rcu_dereference(tun->tfiles[rxq]);
> +		if (tfile)
> +			break;
> +	}
> 
>  out:
> -	netif_tx_unlock_bh(tun->dev);
> -	return err;
> +	return tfile;
>  }
> 
> -static void __tun_detach(struct tun_struct *tun)
> +static int tun_detach(struct tun_file *tfile, bool clean)
>  {
> -	struct tun_file *tfile = tun->tfile;
> -	/* Detach from net device */
> -	netif_tx_lock_bh(tun->dev);
> -	netif_carrier_off(tun->dev);
> -	tun->tfile = NULL;
> -	netif_tx_unlock_bh(tun->dev);
> -
> -	/* Drop read queue */
> -	skb_queue_purge(&tfile->socket.sk->sk_receive_queue);
> -
> -	/* Drop the extra count on the net device */
> -	dev_put(tun->dev);
> -}
> +	struct tun_struct *tun;
> +	struct net_device *dev = NULL;
> +	bool destroy = false;
> 
> -static void tun_detach(struct tun_struct *tun)
> -{
> -	rtnl_lock();
> -	__tun_detach(tun);
> -	rtnl_unlock();
> -}
> +	spin_lock(&tun_lock);
> 
> -static struct tun_struct *__tun_get(struct tun_file *tfile)
> -{
> -	struct tun_struct *tun = NULL;
> +	tun = rcu_dereference_protected(tfile->tun,
> +					lockdep_is_held(&tun_lock));
> +	if (tun) {
> +		int index = tun_get_slot(tun, tfile);

Don't we need to be in an RCU read-side critical section in order to
safely call tun_get_slot()?

Or is the fact that we are calling with tun_lock held sufficient?
If the latter, then the rcu_dereference() in tun_get_slot() should
use rcu_dereference_protected() rather than rcu_dereference().

> +		if (index == -1) {
> +			spin_unlock(&tun_lock);
> +			return -EINVAL;
> +		}
> +		dev = tun->dev;
> +
> +		rcu_assign_pointer(tun->tfiles[index], NULL);
> +		rcu_assign_pointer(tfile->tun, NULL);
> +		--tun->numqueues;
> +		sock_put(&tfile->sk);
> 
> -	if (atomic_inc_not_zero(&tfile->count))
> -		tun = tfile->tun;
> +		if (tun->numqueues == 0 && !(tun->flags & TUN_PERSIST))
> +			destroy = true;
> +	}
> +
> +	spin_unlock(&tun_lock);
> +
> +	synchronize_rcu();
> +	if (clean)
> +		sock_put(&tfile->sk);
> 
> -	return tun;
> +	if (destroy) {
> +		rtnl_lock();
> +		if (dev->reg_state == NETREG_REGISTERED)
> +			unregister_netdevice(dev);
> +		rtnl_unlock();
> +	}
> +
> +	return 0;
>  }
> 
> -static struct tun_struct *tun_get(struct file *file)
> +static void tun_detach_all(struct net_device *dev)
>  {
> -	return __tun_get(file->private_data);
> +	struct tun_struct *tun = netdev_priv(dev);
> +	struct tun_file *tfile, *tfile_list[MAX_TAP_QUEUES];
> +	int i, j = 0;
> +
> +	spin_lock(&tun_lock);
> +
> +	for (i = 0; i < MAX_TAP_QUEUES && tun->numqueues; i++) {
> +		tfile = rcu_dereference_protected(tun->tfiles[i],
> +						lockdep_is_held(&tun_lock));
> +		if (tfile) {
> +			wake_up_all(&tfile->wq.wait);
> +			tfile_list[i++] = tfile;
> +			rcu_assign_pointer(tun->tfiles[i], NULL);
> +			rcu_assign_pointer(tfile->tun, NULL);
> +			--tun->numqueues;
> +		}
> +	}
> +	BUG_ON(tun->numqueues != 0);
> +	spin_unlock(&tun_lock);
> +
> +	synchronize_rcu();
> +	for(--j; j >= 0; j--)
> +		sock_put(&tfile_list[j]->sk);
>  }
> 
> -static void tun_put(struct tun_struct *tun)
> +static int tun_attach(struct tun_struct *tun, struct file *file)
>  {
> -	struct tun_file *tfile = tun->tfile;
> +	struct tun_file *tfile = file->private_data;
> +	int err, index;
> +
> +	ASSERT_RTNL();
> +
> +	spin_lock(&tun_lock);
> 
> -	if (atomic_dec_and_test(&tfile->count))
> -		tun_detach(tfile->tun);
> +	err = -EINVAL;
> +	if (rcu_dereference_protected(tfile->tun, lockdep_is_held(&tun_lock)))
> +		goto out;
> +
> +	err = -EBUSY;
> +	if (!(tun->flags & TUN_TAP_MQ) &&
> +		rcu_dereference_protected(tun->tfiles[0],
> +					lockdep_is_held(&tun_lock))) {
> +		/* Multiqueue is only for TAP */
> +		goto out;
> +	}
> +
> +	if (tun->numqueues == MAX_TAP_QUEUES)
> +		goto out;
> +
> +	err = 0;
> +	index = tun_get_slot(tun, NULL);
> +	BUG_ON(index == -1);
> +	rcu_assign_pointer(tfile->tun, tun);
> +	rcu_assign_pointer(tun->tfiles[index], tfile);
> +	sock_hold(&tfile->sk);
> +	tun->numqueues++;
> +
> +	if (tun->numqueues == 1)
> +		netif_carrier_on(tun->dev);
> +
> +	/* device is allowed to go away first, so no need to hold extra refcnt.	 */
> +out:
> +	spin_unlock(&tun_lock);
> +	return err;
>  }
> 
>  /* TAP filtering */
> @@ -332,16 +438,7 @@ static const struct ethtool_ops tun_ethtool_ops;
>  /* Net device detach from fd. */
>  static void tun_net_uninit(struct net_device *dev)
>  {
> -	struct tun_struct *tun = netdev_priv(dev);
> -	struct tun_file *tfile = tun->tfile;
> -
> -	/* Inform the methods they need to stop using the dev.
> -	 */
> -	if (tfile) {
> -		wake_up_all(&tfile->wq.wait);
> -		if (atomic_dec_and_test(&tfile->count))
> -			__tun_detach(tun);
> -	}
> +	tun_detach_all(dev);
>  }
> 
>  /* Net device open. */
> @@ -361,10 +458,10 @@ static int tun_net_close(struct net_device *dev)
>  /* Net device start xmit */
>  static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
> -	struct tun_struct *tun = netdev_priv(dev);
> -	struct tun_file *tfile = tun->tfile;
> +	struct tun_file *tfile = NULL;
> 
> -	tun_debug(KERN_INFO, tun, "tun_net_xmit %d\n", skb->len);
> +	rcu_read_lock();
> +	tfile = tun_get_queue(dev, skb);
> 
>  	/* Drop packet if interface is not attached */
>  	if (!tfile)
> @@ -381,7 +478,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  		goto drop;
> 
>  	if (skb_queue_len(&tfile->socket.sk->sk_receive_queue) >= dev->tx_queue_len) {
> -		if (!(tun->flags & TUN_ONE_QUEUE)) {
> +		if (!(tfile->flags & TUN_ONE_QUEUE) && !(tfile->flags && TUN_TAP_MQ)) {
>  			/* Normal queueing mode. */
>  			/* Packet scheduler handles dropping of further packets. */
>  			netif_stop_queue(dev);
> @@ -390,7 +487,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  			 * error is more appropriate. */
>  			dev->stats.tx_fifo_errors++;
>  		} else {
> -			/* Single queue mode.
> +			/* Single queue mode or multi queue mode.
>  			 * Driver handles dropping of all packets itself. */
>  			goto drop;
>  		}
> @@ -408,9 +505,11 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>  		kill_fasync(&tfile->fasync, SIGIO, POLL_IN);
>  	wake_up_interruptible_poll(&tfile->wq.wait, POLLIN |
>  				   POLLRDNORM | POLLRDBAND);
> +	rcu_read_unlock();
>  	return NETDEV_TX_OK;
> 
>  drop:
> +	rcu_read_unlock();
>  	dev->stats.tx_dropped++;
>  	kfree_skb(skb);
>  	return NETDEV_TX_OK;
> @@ -526,16 +625,22 @@ static void tun_net_init(struct net_device *dev)
>  static unsigned int tun_chr_poll(struct file *file, poll_table * wait)
>  {
>  	struct tun_file *tfile = file->private_data;
> -	struct tun_struct *tun = __tun_get(tfile);
> +	struct tun_struct *tun = NULL;
>  	struct sock *sk;
>  	unsigned int mask = 0;
> 
> -	if (!tun)
> +	if (!tfile)
>  		return POLLERR;
> 
> -	sk = tfile->socket.sk;
> +	rcu_read_lock();
> +	tun = rcu_dereference(tfile->tun);
> +	if (!tun) {
> +		rcu_read_unlock();
> +		return POLLERR;
> +	}
> +	rcu_read_unlock();
> 
> -	tun_debug(KERN_INFO, tun, "tun_chr_poll\n");
> +	sk = &tfile->sk;
> 
>  	poll_wait(file, &tfile->wq.wait, wait);
> 
> @@ -547,10 +652,12 @@ static unsigned int tun_chr_poll(struct file *file, poll_table * wait)
>  	     sock_writeable(sk)))
>  		mask |= POLLOUT | POLLWRNORM;
> 
> -	if (tun->dev->reg_state != NETREG_REGISTERED)
> +	rcu_read_lock();
> +	tun = rcu_dereference(tfile->tun);
> +	if (!tun || tun->dev->reg_state != NETREG_REGISTERED)
>  		mask = POLLERR;
> +	rcu_read_unlock();
> 
> -	tun_put(tun);
>  	return mask;
>  }
> 
> @@ -706,8 +813,10 @@ static ssize_t tun_get_user(struct tun_file *tfile,
>  		skb_shinfo(skb)->gso_segs = 0;
>  	}
> 
> -	tun = __tun_get(tfile);
> +	rcu_read_lock();
> +	tun = rcu_dereference(tfile->tun);
>  	if (!tun) {
> +		rcu_read_unlock();
>  		return -EBADFD;
>  	}
> 
> @@ -722,7 +831,7 @@ static ssize_t tun_get_user(struct tun_file *tfile,
> 
>  	tun->dev->stats.rx_packets++;
>  	tun->dev->stats.rx_bytes += len;
> -	tun_put(tun);
> +	rcu_read_unlock();
> 
>  	netif_rx_ni(skb);
> 
> @@ -732,16 +841,17 @@ err_free:
>  	count = -EINVAL;
>  	kfree_skb(skb);
>  err:
> -	tun = __tun_get(tfile);
> +	rcu_read_lock();
> +	tun = rcu_dereference(tfile->tun);
>  	if (!tun) {
> +		rcu_read_unlock();
>  		return -EBADFD;
>  	}
> -
>  	if (drop)
>  		tun->dev->stats.rx_dropped++;
>  	if (error)
>  		tun->dev->stats.rx_frame_errors++;
> -	tun_put(tun);
> +	rcu_read_unlock();
>  	return count;
>  }
> 
> @@ -834,12 +944,13 @@ static ssize_t tun_put_user(struct tun_file *tfile,
>  	skb_copy_datagram_const_iovec(skb, 0, iv, total, len);
>  	total += skb->len;
> 
> -	tun = __tun_get(tfile);
> +	rcu_read_lock();
> +	tun = rcu_dereference(tfile->tun);
>  	if (tun) {
>  		tun->dev->stats.tx_packets++;
>  		tun->dev->stats.tx_bytes += len;
> -		tun_put(tun);
>  	}
> +	rcu_read_unlock();
> 
>  	return total;
>  }
> @@ -869,28 +980,31 @@ static ssize_t tun_do_read(struct tun_file *tfile,
>  				break;
>  			}
> 
> -			tun = __tun_get(tfile);
> +			rcu_read_lock();
> +			tun = rcu_dereference(tfile->tun);
>  			if (!tun) {
> -				ret = -EIO;
> +				ret = -EBADFD;
> +				rcu_read_unlock();
>  				break;
>  			}
>  			if (tun->dev->reg_state != NETREG_REGISTERED) {
>  				ret = -EIO;
> -				tun_put(tun);
> +				rcu_read_unlock();
>  				break;
>  			}
> -			tun_put(tun);
> +			rcu_read_unlock();
> 
>  			/* Nothing to read, let's sleep */
>  			schedule();
>  			continue;
>  		}
> 
> -		tun = __tun_get(tfile);
> +		rcu_read_lock();
> +		tun = rcu_dereference(tfile->tun);
>  		if (tun) {
>  			netif_wake_queue(tun->dev);
> -			tun_put(tun);
>  		}
> +		rcu_read_unlock();
> 
>  		ret = tun_put_user(tfile, skb, iv, len);
>  		kfree_skb(skb);
> @@ -1030,6 +1144,9 @@ static int tun_flags(struct tun_struct *tun)
>  	if (tun->flags & TUN_VNET_HDR)
>  		flags |= IFF_VNET_HDR;
> 
> +	if (tun->flags & TUN_TAP_MQ)
> +		flags |= IFF_MULTI_QUEUE;
> +
>  	return flags;
>  }
> 
> @@ -1109,6 +1226,10 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>  			/* TAP device */
>  			flags |= TUN_TAP_DEV;
>  			name = "tap%d";
> +			if (ifr->ifr_flags & IFF_MULTI_QUEUE) {
> +				flags |= TUN_TAP_MQ;
> +				name = "mqtap%d";
> +			}
>  		} else
>  			return -EINVAL;
> 
> @@ -1134,6 +1255,8 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>  		dev->hw_features = NETIF_F_SG | NETIF_F_FRAGLIST |
>  			TUN_USER_FEATURES;
>  		dev->features = dev->hw_features;
> +		if (ifr->ifr_flags & IFF_MULTI_QUEUE)
> +			dev->features |= NETIF_F_LLTX;
> 
>  		err = register_netdevice(tun->dev);
>  		if (err < 0)
> @@ -1166,6 +1289,11 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>  	else
>  		tun->flags &= ~TUN_VNET_HDR;
> 
> +	if (ifr->ifr_flags & IFF_MULTI_QUEUE)
> +		tun->flags |= TUN_TAP_MQ;
> +	else
> +		tun->flags &= ~TUN_TAP_MQ;
> +
>  	/* Cache flags from tun device */
>  	tfile->flags = tun->flags;
>  	/* Make sure persistent devices do not get stuck in
> @@ -1256,38 +1384,39 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
>  				(unsigned int __user*)argp);
>  	}
> 
> -	rtnl_lock();
> -
> -	tun = __tun_get(tfile);
> -	if (cmd == TUNSETIFF && !tun) {
> +	ret = 0;
> +	if (cmd == TUNSETIFF) {
> +		rtnl_lock();
>  		ifr.ifr_name[IFNAMSIZ-1] = '\0';
> -
>  		ret = tun_set_iff(tfile->net, file, &ifr);
> -
> +		rtnl_unlock();
>  		if (ret)
> -			goto unlock;
> -
> +			return ret;
>  		if (copy_to_user(argp, &ifr, ifreq_len))
> -			ret = -EFAULT;
> -		goto unlock;
> +			return -EFAULT;
> +		return ret;
>  	}
> 
> +	rtnl_lock();
> +
> +	rcu_read_lock();
> +
>  	ret = -EBADFD;
> +	tun = rcu_dereference(tfile->tun);
>  	if (!tun)
>  		goto unlock;
> 
> -	tun_debug(KERN_INFO, tun, "tun_chr_ioctl cmd %d\n", cmd);
> 
> -	ret = 0;
> -	switch (cmd) {
> +	switch(cmd) {
>  	case TUNGETIFF:
>  		ret = tun_get_iff(current->nsproxy->net_ns, tun, &ifr);
> +		rcu_read_unlock();
>  		if (ret)
> -			break;
> +			goto out;
> 
>  		if (copy_to_user(argp, &ifr, ifreq_len))
>  			ret = -EFAULT;
> -		break;
> +		goto out;
> 
>  	case TUNSETNOCSUM:
>  		/* Disable/Enable checksum */
> @@ -1349,9 +1478,10 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
>  		/* Get hw address */
>  		memcpy(ifr.ifr_hwaddr.sa_data, tun->dev->dev_addr, ETH_ALEN);
>  		ifr.ifr_hwaddr.sa_family = tun->dev->type;
> +		rcu_read_unlock();
>  		if (copy_to_user(argp, &ifr, ifreq_len))
>  			ret = -EFAULT;
> -		break;
> +		goto out;
> 
>  	case SIOCSIFHWADDR:
>  		/* Set hw address */
> @@ -1367,9 +1497,9 @@ static long __tun_chr_ioctl(struct file *file, unsigned int cmd,
>  	}
> 
>  unlock:
> +	rcu_read_unlock();
> +out:
>  	rtnl_unlock();
> -	if (tun)
> -		tun_put(tun);
>  	return ret;
>  }
> 
> @@ -1541,31 +1671,8 @@ static int tun_chr_open(struct inode *inode, struct file * file)
>  static int tun_chr_close(struct inode *inode, struct file *file)
>  {
>  	struct tun_file *tfile = file->private_data;
> -	struct tun_struct *tun;
> -
> -	tun = __tun_get(tfile);
> -	if (tun) {
> -		struct net_device *dev = tun->dev;
> -
> -		tun_debug(KERN_INFO, tun, "tun_chr_close\n");
> -
> -		__tun_detach(tun);
> -
> -		/* If desirable, unregister the netdevice. */
> -		if (!(tun->flags & TUN_PERSIST)) {
> -			rtnl_lock();
> -			if (dev->reg_state == NETREG_REGISTERED)
> -				unregister_netdevice(dev);
> -			rtnl_unlock();
> -		}
> -
> -		/* drop the reference that netdevice holds */
> -		sock_put(&tfile->sk);
> -
> -	}
> 
> -	/* drop the reference that file holds */
> -	sock_put(&tfile->sk);
> +	tun_detach(tfile, true);
> 
>  	return 0;
>  }
> @@ -1693,14 +1800,17 @@ static void tun_cleanup(void)
>   * holding a reference to the file for as long as the socket is in use. */
>  struct socket *tun_get_socket(struct file *file)
>  {
> -	struct tun_struct *tun;
> +	struct tun_struct *tun = NULL;
>  	struct tun_file *tfile = file->private_data;
>  	if (file->f_op != &tun_fops)
>  		return ERR_PTR(-EINVAL);
> -	tun = tun_get(file);
> -	if (!tun)
> +	rcu_read_lock();
> +	tun = rcu_dereference(tfile->tun);
> +	if (!tun) {
> +		rcu_read_unlock();
>  		return ERR_PTR(-EBADFD);
> -	tun_put(tun);
> +	}
> +	rcu_read_unlock();
>  	return &tfile->socket;
>  }
>  EXPORT_SYMBOL_GPL(tun_get_socket);
> 
> --
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