vhost_net: correctly check tx avail during rx busy polling

BugLink: http://bugs.launchpad.net/bugs/1718412
[ Upstream commit 8b949bef9172ca69d918e93509a4ecb03d0355e0 ]

We check tx avail through vhost_enable_notify() in the past which is
wrong since it only checks whether or not guest has filled more
available buffer since last avail idx synchronization which was just
done by vhost_vq_avail_empty() before. What we really want is checking
pending buffers in the avail ring. Fix this by calling
vhost_vq_avail_empty() instead.

This issue could be noticed by doing netperf TCP_RR benchmark as
client from guest (but not host). With this fix, TCP_RR from guest to
localhost restores from 1375.91 trans per sec to 55235.28 trans per
sec on my laptop (Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz).

Fixes: 030881372460 ("vhost_net: basic polling support")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 06d044862e589ff05194d695bfdc610ed7e62195..1c75572f5a3fb934f59057572bef114b5557dd42 100644 (file)
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -634,8 +634,13 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk)
 
                preempt_enable();
 
-               if (vhost_enable_notify(&net->dev, vq))
+               if (!vhost_vq_avail_empty(&net->dev, vq))
                        vhost_poll_queue(&vq->poll);
+               else if (unlikely(vhost_enable_notify(&net->dev, vq))) {
+                       vhost_disable_notify(&net->dev, vq);
+                       vhost_poll_queue(&vq->poll);
+               }
+
                mutex_unlock(&vq->mutex);
 
                len = peek_head_len(rvq, sk);