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

["Xie, Huawei" <huawei.xie@intel.com>]

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.