netdev - RE: [PATCH net] vhost_net: don't continue to call the recvmsg when meet errors

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Date:   Thu, 1 Dec 2016 04:41:40 +0000
From:   wangyunjian <wangyunjian@...wei.com>
To:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        caihe <caihe@...wei.com>
Subject: RE: [PATCH net] vhost_net: don't continue to call the recvmsg when
 meet errors

>-----Original Message-----
>From: Jason Wang [mailto:jasowang@...hat.com] 
>Sent: Thursday, December 01, 2016 11:37 AM
>To: Michael S. Tsirkin
>Cc: wangyunjian; netdev@...r.kernel.org; linux-kernel@...r.kernel.org; caihe
>Subject: Re: [PATCH net] vhost_net: don't continue to call the recvmsg when meet errors
>
>
>
>On 2016年12月01日 11:27, Michael S. Tsirkin wrote:
>> On Thu, Dec 01, 2016 at 11:26:21AM +0800, Jason Wang wrote:
>>> >
>>> >
>>> >On 2016年12月01日 11:21, Michael S. Tsirkin wrote:
>>>> > >On Thu, Dec 01, 2016 at 02:48:59AM +0000, wangyunjian wrote:
>>>>>> > > > >-----Original Message-----
>>>>>> > > > >From: Michael S. Tsirkin [mailto:mst@...hat.com]
>>>>>> > > > >Sent: Wednesday, November 30, 2016 9:41 PM
>>>>>> > > > >To: wangyunjian
>>>>>> > > > >Cc:jasowang@...hat.com;netdev@...r.kernel.org;linux-kernel@
>>>>>> > > > >vger.kernel.org; caihe
>>>>>> > > > >Subject: Re: [PATCH net] vhost_net: don't continue to call 
>>>>>> > > > >the recvmsg when meet errors
>>>>>> > > > >
>>>>>> > > > >On Wed, Nov 30, 2016 at 08:10:57PM +0800, Yunjian Wang wrote:
>>>>>>> > > > > >When we meet an error(err=-EBADFD) recvmsg,
>>>>>> > > > >How do you get EBADFD? Won't vhost_net_rx_peek_head_len 
>>>>>> > > > >return 0 in this case, breaking the loop?
>>>>> > > >We started many guest VMs while attaching/detaching some virtio-net nics for loop.
>>>>> > > >The soft lockup might happened. The err is -EBADFD.
>>>>> > > >
>>>> > >OK, I'd like to figure out what happened here. why don't we get 0 
>>>> > >when we peek at the head?
>>>> > >
>>>> > >EBADFD is from here:
>>>> > >          struct tun_struct *tun = __tun_get(tfile); ...
>>>> > >          if (!tun)
>>>> > >                  return -EBADFD;
>>>> > >
>>>> > >but then:
>>>> > >static int tun_peek_len(struct socket *sock) {
>>>> > >
>>>> > >...
>>>> > >
>>>> > >          struct tun_struct *tun; ...
>>>> > >          tun = __tun_get(tfile);
>>>> > >          if (!tun)
>>>> > >                  return 0;
>>>> > >
>>>> > >
>>>> > >so peek len should return 0.
>>>> > >
>>>> > >then while will exit:
>>>> > >          while ((sock_len = vhost_net_rx_peek_head_len(net, 
>>>> > >sock->sk))) ...
>>>> > >
>>> >
>>> >Consider this case: user do ip link del link tap0 before recvmsg() 
>>> >but after
>>> >tun_peek_len() ?
>> Sure, this can happen, but I think we'll just exit on the next loop, 
>> won't we?
>>
>
>Right, this is the only case I can image for -EBADFD, let's wait for the author to the steps.
>

Thanks, I understand it don't happen in the latest kernel version.
My problem happened using kernel version 3.10.0-xx

The peek len willn't return 0.

static int peek_head_len(struct sock *sk)
{
	struct sk_buff *head;
	int len = 0;
	unsigned long flags;

	spin_lock_irqsave(&sk->sk_receive_queue.lock, flags);
	head = skb_peek(&sk->sk_receive_queue);
	if (likely(head)) {
		len = head->len;
		if (skb_vlan_tag_present(head))
			len += VLAN_HLEN;
	}

	spin_unlock_irqrestore(&sk->sk_receive_queue.lock, flags);
	return len;
}