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

[Jason Wang <jasowang@redhat.com>]

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@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:
>>>> 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() ?

>> meesage log:
>> kernel:[609608.510180]BUG: soft lockup - CPU#18 stuck for 23s! [vhost-60898:126093]
>> call trace:
>> [<fffffffa0132967>]vhost_get_vq_desc+0x1e7/0x984 [vhost]
>> [<fffffffa02037e6>]handle_rx+0x226/0x810 [vhost_net]
>> [<fffffffa0203de5>]handle_rx_net+0x15/0x20 [vhost_net]
>> [<fffffffa013160b>]vhost_worker+0xfb/0x1e0 [vhost]
>> [<fffffffa0131510>]? vhost_dev_reset_owner+0x50/0x50 [vhost]
>> [<fffffff810a5c7f>]kthread+0xcf/0xe0
>> [<fffffff810a5bb0>]? kthread_create_on_node+0x140/0x140
>> [<fffffff81648898>]ret_from_fork+0x58/0x90
>> [<fffffff810a5bb0>]? kthread_create_on_node+0x140/0x140
> So somehow you keep seeing something in tun when we peek.
> IMO this is the problem we should try to fix.
> Could you try debugging the root cause here?
>
>>>> the error handling in vhost
>>>> handle_rx() will continue. This will cause a soft CPU lockup in vhost thread.
>>>>
>>>> Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
>>>> ---
>>>>   drivers/vhost/net.c | 3 +++
>>>>   1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>>> index 5dc128a..edc470b 100644
>>>> --- a/drivers/vhost/net.c
>>>> +++ b/drivers/vhost/net.c
>>>> @@ -717,6 +717,9 @@ static void handle_rx(struct vhost_net *net)
>>>>   			pr_debug("Discarded rx packet: "
>>>>   				 " len %d, expected %zd\n", err, sock_len);
>>>>   			vhost_discard_vq_desc(vq, headcount);
>>>> +			/* Don't continue to do, when meet errors. */
>>>> +			if (err < 0)
>>>> +				goto out;
>>> You might get e.g. EAGAIN and I think you need to retry
>>> in this case.
>>>
>>>>   			continue;
>>>>   		}
>>>>   		/* Supply virtio_net_hdr if VHOST_NET_F_VIRTIO_NET_HDR */
>>>> -- 
>>>> 1.9.5.msysgit.1
>>>>