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

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

On Thu, Jun 02, 2016 at 08:39:36AM +0000, Xie, Huawei wrote:
> On 6/1/2016 2:03 PM, Yuanhan Liu wrote:
> > On Wed, Jun 01, 2016 at 05:40:08AM +0000, Xie, Huawei wrote:
> >> On 5/30/2016 4:20 PM, Yuanhan Liu wrote:
> >>> 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
> >>>  
> >>>
> >> Yes, seems it has same issue though haven't confirmed with asm code.
> > So, move the "volatile" qualifier to VIRTQUEUE_NUSED?
> >
> > 	--yliu
> >
> 
> Yes, anyway this is just intermediate fix. In next patch, will declare
> the idx as volatile, and remove the qualifier in the macro.

Hmm.., why we need an intermediate fix then, if we can come up with an
ultimate fix very quickly?

	--yliu