linux-kernel - Re: [PATCH V4 3/3] tuntap: allow polling/writing/reading when detached

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Date:	Wed, 16 Jan 2013 14:17:06 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V4 3/3] tuntap: allow polling/writing/reading when detached

On Tuesday, January 15, 2013 06:48:08 PM Michael S. Tsirkin wrote:
> 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 <jasowang@...hat.com>
> > ---
> > 
> >  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?

I miss this, something is needed.
> 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.

Correct, will post another version, thanks.
> 
> > @@ -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);
> 
> --
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