[PATCH 026/104] vhost_net: poll vhost queue after marking DMA is done

[Kamal Mostafa]

3.8.13.11 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jason Wang <jasowang at redhat.com>

[ Upstream commit 19c73b3e08d16ee923f3962df4abf6205127896a ]

We used to poll vhost queue before making DMA is done, this is racy if vhost
thread were waked up before marking DMA is done which can result the signal to
be missed. Fix this by always polling the vhost thread before DMA is done.

Signed-off-by: Jason Wang <jasowang at redhat.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
[ kamal: backport to 3.8 ]
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/vhost/net.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index dfff647..b8dfb94 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -210,6 +210,11 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
 	struct vhost_virtqueue *vq = ubufs->vq;
 	int cnt = atomic_read(&ubufs->kref.refcount);
 
+	/* set len to mark this desc buffers done DMA */
+	vq->heads[ubuf->desc].len = success ?
+		VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN;
+	vhost_ubuf_put(ubufs);
+
 	/*
 	 * Trigger polling thread if guest stopped submitting new buffers:
 	 * in this case, the refcount after decrement will eventually reach 1
@@ -220,10 +225,6 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
 	 */
 	if (cnt <= 2 || !(cnt % 16))
 		vhost_poll_queue(&vq->poll);
-	/* set len to mark this desc buffers done DMA */
-	vq->heads[ubuf->desc].len = success ?
-		VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN;
-	vhost_ubuf_put(ubufs);
 }
 
 /* Expects to be always run from workqueue - which acts as
-- 
1.8.1.2