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virtio_net: fix race in RX VQ processing

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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>
This commit is contained in:
Michael S. Tsirkin 2013-07-09 08:13:04 +03:00 committed by David S. Miller
parent cc229884d3
commit cbdadbbf0c
1 changed files with 3 additions and 2 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

5
drivers/net/virtio_net.c
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@ -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 @@ static int virtnet_poll(struct napi_struct *napi, int budget)
/* 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);