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

From: Jason Wang
Date: Wed Nov 30 2016 - 22:13:24 EST

On 2016å12æ01æ 10:48, 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.

How did you do the attaching/detaching? AFAIK, the -EBADFD can only happens when you deleting tun device during vhost_net transmission.

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]
[<fffffff810a5bb0>]? kthread_create_on_node+0x140/0x140
[<fffffff810a5bb0>]? kthread_create_on_node+0x140/0x140

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@xxxxxxxxxx>
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.

/* Supply virtio_net_hdr if VHOST_NET_F_VIRTIO_NET_HDR */