Patch "vhost_net: correctly check tx avail during rx busy polling" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

vhost_net: correctly check tx avail during rx busy polling

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
vhost_net-correctly-check-tx-avail-during-rx-busy-polling.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jason Wang <[email protected]>
Date: Tue, 5 Sep 2017 09:22:05 +0800
Subject: vhost_net: correctly check tx avail during rx busy polling

From: Jason Wang <[email protected]>

[ 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).

---
 drivers/vhost/net.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

@@ -537,8 +537,13 @@ static int vhost_net_rx_peek_head_len(st
 
 		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(sk);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/vhost_net-correctly-check-tx-avail-during-rx-busy-polling.patch