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:

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On 2016年12月01日 11:21, Michael S. Tsirkin wrote:

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On Thu, Dec 01, 2016 at 02:48:59AM +0000, wangyunjian wrote:

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-----Original Message-----
From: Michael S. Tsirkin [mailto:mst@redhat.com]
Sent: Wednesday, November 30, 2016 9:41 PM
To: wangyunjian
Cc:jasowang@redhat.com;netdev@vger.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:

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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.