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

[Jason Wang]

On 2016å12æ01æ 11:27, Michael S. Tsirkin wrote:
> On Thu, Dec 01, 2016 at 11:26:21AM +0800, Jason Wang wrote:
> > >
> > >
> > >On 2016å12æ01æ 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@xxxxxxxxxx]
> > > > > > > > >Sent: Wednesday, November 30, 2016 9:41 PM
> > > > > > > > >To: wangyunjian
> > > > > > > > >Cc:jasowang@xxxxxxxxxx;netdev@xxxxxxxxxxxxxxx;linux-kernel@xxxxxxxxxxxxxxx; 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.