virtio_net: fix race in RX VQ processing

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@redhat.com>
Tested-by: Jason Wang <jasowang@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index c9e00387d9996e3c5b0660a57bf296e4ff15f3d5..42d670a468f89462a4e31ebde531c6a0e9c3b644 100644 (file)
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -602,7 +602,7 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
                container_of(napi, struct receive_queue, napi);
        struct virtnet_info *vi = rq->vq->vdev->priv;
        void *buf;
-       unsigned int len, received = 0;
+       unsigned int r, len, received = 0;
 
 again:
        while (received < budget &&
@@ -619,8 +619,9 @@ again:
 
        /* Out of packets? */
        if (received < budget) {
+               r = virtqueue_enable_cb_prepare(rq->vq);
                napi_complete(napi);
-               if (unlikely(!virtqueue_enable_cb(rq->vq)) &&
+               if (unlikely(virtqueue_poll(rq->vq, r)) &&
                    napi_schedule_prep(napi)) {
                        virtqueue_disable_cb(rq->vq);
                        __napi_schedule(napi);