Re: [dpdk-dev] [PATCH] virtio: use volatile to get used->idx in the loop

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Wed, May 25, 2016 at 12:16:41AM +0800, Huawei Xie wrote:
> There is no external function call or any barrier in the loop,
> the used->idx would only be retrieved once.
> 
> Signed-off-by: Huawei Xie <huawei.xie@intel.com>
> ---
>  drivers/net/virtio/virtio_ethdev.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio/virtio_ethdev.c b/drivers/net/virtio/virtio_ethdev.c
> index c3fb628..f6d6305 100644
> --- a/drivers/net/virtio/virtio_ethdev.c
> +++ b/drivers/net/virtio/virtio_ethdev.c
> @@ -204,7 +204,8 @@ virtio_send_command(struct virtqueue *vq, struct virtio_pmd_ctrl *ctrl,
>  		usleep(100);
>  	}
>  
> -	while (vq->vq_used_cons_idx != vq->vq_ring.used->idx) {
> +	while (vq->vq_used_cons_idx !=
> +	       *((volatile uint16_t *)(&vq->vq_ring.used->idx))) {

I'm wondering maybe we could fix VIRTQUEUE_NUSED (which has no such
qualifier) and use this macro here?

If you check the reference of that macro, you might find similar
issues, say, it is also used inside the while-loop of
virtio_recv_mergeable_pkts().

	--yliu