Subject: Re: [PATCH v2] net-tun: restructure tun_do_read for better sleep/wakeup efficiency

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 20 May 2014 09:22:32 +0300

On Tue, May 20, 2014 at 12:51:40PM +0800, Jason Wang wrote:
> On 05/20/2014 12:06 AM, Michael S. Tsirkin wrote:
> > On Mon, May 19, 2014 at 05:27:41PM +0800, Jason Wang wrote:
> >>> @@ -1330,47 +1329,26 @@ done:
> >>>  static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
> >>>  			   const struct iovec *iv, ssize_t len, int noblock)
> >>>  {
> >>> -	DECLARE_WAITQUEUE(wait, current);
> >>>  	struct sk_buff *skb;
> >>>  	ssize_t ret = 0;
> >>> +	int peeked, err, off = 0;
> >>>  
> >>>  	tun_debug(KERN_INFO, tun, "tun_do_read\n");
> >>>  
> >>> -	if (unlikely(!noblock))
> >>> -		add_wait_queue(&tfile->wq.wait, &wait);
> >>> -	while (len) {
> >>> -		if (unlikely(!noblock))
> >>> -			current->state = TASK_INTERRUPTIBLE;
> >>> +	if (!len)
> >>> +		return ret;
> >>>  
> >>> -		/* Read frames from the queue */
> >>> -		if (!(skb = skb_dequeue(&tfile->socket.sk->sk_receive_queue))) {
> >>> -			if (noblock) {
> >>> -				ret = -EAGAIN;
> >>> -				break;
> >>> -			}
> >>> -			if (signal_pending(current)) {
> >>> -				ret = -ERESTARTSYS;
> >>> -				break;
> >>> -			}
> >>> -			if (tun->dev->reg_state != NETREG_REGISTERED) {
> >>> -				ret = -EIO;
> >>> -				break;
> >>> -			}
> >>> -
> >>> -			/* Nothing to read, let's sleep */
> >>> -			schedule();
> >>> -			continue;
> >>> -		}
> >>> +	if (tun->dev->reg_state != NETREG_REGISTERED)
> >>> +		return -EIO;
> >>>  
> >>> +	/* Read frames from queue */
> >>> +	skb = __skb_recv_datagram(tfile->socket.sk, noblock ? MSG_DONTWAIT : 0,
> >>> +				  &peeked, &off, &err);
> >>> +	if (skb) {
> >> Still a little bit difference. We check the reg_state after we're sure
> >> there's nothing left in sk_receive_queue. But this patch returns -EIO
> >> before trying to dequeue skb.
> >
> > Yes but what's the concern here? What does userspace do
> > to notice the change in behaviour?
> > tun_detach calls tun_queue_purge before unregister_netdevice
> > so apparently there's never anything in queue when
> > state isn't registered.
> > Did I miss something?
> >
> 
> >From rollback_registered_many(), ndo_uninit() was called after reg_state
> was changed to NETREG_UNREGISTERING. So userspace could read something
> in the small window. Not sure it was a problem, but it does have minor
> difference.

OK so userspace reads the fd when it's destroyed.
Previously it could get EIO, now it can get EIO.
Previously packets could get dropped, now packets could get dropped.
Sure timing changed but can the change break some userspace? How?

-- 
MSt