Subject: Re: [PATCH 2/2] virtio_net: fix race in RX VQ processing

From: Jason Wang <[email-masked]>
Date: Tue, 09 Jul 2013 11:28:34 +0800

On 07/08/2013 05:04 PM, Michael S. Tsirkin wrote:
> 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 <[email-masked]>
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---

Tested-by: Jason Wang <[email-masked]>
Acked-by: Jason Wang <[email-masked]>
>  drivers/net/virtio_net.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 5305bd1..fbdd79a 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -622,8 +622,9 @@ again:
>  
>  	/* Out of packets? */
>  	if (received < budget) {
> +		unsigned 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);