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

From: Jason Wang <[email-masked]>
Date: Tue, 20 May 2014 14:40:46 +0800

On 05/20/2014 02:22 PM, Michael S. Tsirkin wrote:
> 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?
>

Probably not even noticeable by userspace. Though it was not hard to
keep this behaviour, I'm ok with the changes here.