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Subject[ 38/39] virtio_net: fix race in RX VQ processing
3.4-stable review patch.  If anyone has any objections, please let me know.


From: Michael S. Tsirkin <>

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 <>
Tested-by: Jason Wang <>
Acked-by: Jason Wang <>
Signed-off-by: Michael S. Tsirkin <>
Signed-off-by: David S. Miller <>
[wg: Backported to 3.2]
Signed-off-by: Wolfram Gloger <>
Signed-off-by: Greg Kroah-Hartman <>
drivers/net/virtio_net.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -518,7 +518,7 @@ static int virtnet_poll(struct napi_stru
struct virtnet_info *vi = container_of(napi, struct virtnet_info, napi);
void *buf;
- unsigned int len, received = 0;
+ unsigned int r, len, received = 0;

while (received < budget &&
@@ -535,8 +535,9 @@ again:

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

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