Subject: Re: [PATCH V4 3/3] tuntap: allow polling/writing/reading when detached

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 15 Jan 2013 18:48:08 +0200

On Tue, Jan 15, 2013 at 06:28:46PM +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,
>   vhost/qemu needs to to its own cleanup which is complex. We can do this simply
>   by allowing a user can still write to an disabled queue to handle this. And
>   user can still do read but just nothing returned.
> - 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)
> 
> Signed-off-by: Jason Wang <[email-masked]>
> ---
>  drivers/net/tun.c |   18 +++++++++++-------
>  1 files changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index af372d0..eb68937 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -307,7 +307,7 @@ static void tun_flow_update(struct tun_struct *tun, u32 rxhash,
>  
>  	rcu_read_lock();
>  
> -	if (tun->numqueues == 1)
> +	if (tun->numqueues == 1 || queue_index >= tun->numqueues)
>  		goto unlock;
>  
>  	e = tun_flow_find(head, rxhash);

Hmm I don't understand - does something ensure that queue_index is >
numqueues if the queue is disabled?
Can we check tun->disabled and skip tun_flow_update completely?
We might need to switch to rcu_assign for tun->disabled for this
but otherwise it looks easy.

> @@ -406,21 +406,21 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
>  
>  	tun = rtnl_dereference(tfile->tun);
>  
> -	if (tun) {
> +	if (tun && !tfile->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();
> @@ -465,6 +465,10 @@ 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();
> @@ -491,7 +495,7 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
>  	int err;
>  
>  	err = -EINVAL;
> -	if (rtnl_dereference(tfile->tun))
> +	if (rtnl_dereference(tfile->tun) && !tfile->detached)
>  		goto out;
>  
>  	err = -EBUSY;
> @@ -1795,7 +1799,7 @@ 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) || tfile->detached)
>  			ret = -EINVAL;
>  		else
>  			__tun_detach(tfile, false);
> -- 
> 1.7.1