Subject: Re: [PATCH V5 3/3] tuntap: allow polling/writing/reading when detached

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 16 Jan 2013 12:59:11 +0200

On Wed, Jan 16, 2013 at 06:34:01PM +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]>

NAK

> ---
>  drivers/net/tun.c |   43 +++++++++++++++++++++++++------------------
>  1 files changed, 25 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index c81680d..3f011e0 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 || !rtnl_dereference(tfile->detached))
>  		goto unlock;
>  
>  	e = tun_flow_find(head, rxhash);

Did you try to run this with lockdep enabled?
tun_flow_update is called from tun_get_user without rtnl so
rtnl_dereference is arguably wrong, and will cause a lockdep warning.

> @@ -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;
>  }
>  
> @@ -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