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

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

On Thu, Jun 02, 2016 at 08:54:38AM +0000, Xie, Huawei wrote:
> On 6/2/2016 4:52 PM, Yuanhan Liu wrote:
> > 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
> >
> ... Either is OK. I have no preference.

Mind to send an ultimate fix then?

	--yliu