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

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

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.