[ 3.5.y.z extended stable ] Patch "virtio_net: fix race in RX VQ processing" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    virtio_net: fix race in RX VQ processing

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From d1b89c1bfc90124357b663ab6f5b9cc02eb6c629 Mon Sep 17 00:00:00 2001
From: "Michael S. Tsirkin" <mst at redhat.com>
Date: Fri, 2 Aug 2013 14:31:53 +0100
Subject: [PATCH] virtio_net: fix race in RX VQ processing

commit cbdadbbf0c790f79350a8f36029208944c5487d0 upstream

virtio net called virtqueue_enable_cq on RX path after napi_complete, so
with NAPI_STATE_SCHED clear - outside the implicit napi lock.
This violates the requirement to synchronize virtqueue_enable_cq wrt
virtqueue_add_buf.  In particular, used event can move backwards,
causing us to lose interrupts.
In a debug build, this can trigger panic within START_USE.

Jason Wang reports that he can trigger the races artificially,
by adding udelay() in virtqueue_enable_cb() after virtio_mb().

However, we must call napi_complete to clear NAPI_STATE_SCHED before
polling the virtqueue for used buffers, otherwise napi_schedule_prep in
a callback will fail, causing us to lose RX events.

To fix, call virtqueue_enable_cb_prepare with NAPI_STATE_SCHED
set (under napi lock), later call virtqueue_poll with
NAPI_STATE_SCHED clear (outside the lock).

Reported-by: Jason Wang <jasowang at redhat.com>
Tested-by: Jason Wang <jasowang at redhat.com>
Acked-by: Jason Wang <jasowang at redhat.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
[wg: Backported to 3.2]
Signed-off-by: Wolfram Gloger <wmglo at dent.med.uni-muenchen.de>
Signed-off-by: Luis Henriques<luis.henriques at canonical.com>
---
 drivers/net/virtio_net.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index f18149a..858a8da 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -528,7 +528,7 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
 {
 	struct virtnet_info *vi = container_of(napi, struct virtnet_info, napi);
 	void *buf;
-	unsigned int len, received = 0;
+	unsigned int r, len, received = 0;

 again:
 	while (received < budget &&
@@ -545,8 +545,9 @@ again:

 	/* Out of packets? */
 	if (received < budget) {
+		r = virtqueue_enable_cb_prepare(vi->rvq);
 		napi_complete(napi);
-		if (unlikely(!virtqueue_enable_cb(vi->rvq)) &&
+		if (unlikely(virtqueue_poll(vi->rvq, r)) &&
 		    napi_schedule_prep(napi)) {
 			virtqueue_disable_cb(vi->rvq);
 			__napi_schedule(napi);
--
1.8.3.2