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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, May 25, 2016 at 10:47:30AM +0100, Bruce Richardson wrote:
> On Wed, May 25, 2016 at 11:34:24AM +0300, Michael S. Tsirkin wrote:
> > On Wed, May 25, 2016 at 08:25:20AM +0000, Xie, Huawei wrote:
> > > On 5/25/2016 4:12 PM, Xie, Huawei 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))) {
> > > >  		uint32_t idx, desc_idx, used_idx;
> > > >  		struct vring_used_elem *uep;
> > > >  
> > > 
> > > Find this issue when do the code rework of RX/TX queue.
> > > As in other places, we also have loop retrieving the value of avial->idx
> > > or used->idx, i prefer to declare the index in vq structure as volatile
> > > to avoid potential issue.
> 
> Is there a reason why the value is not always volatile? I would have thought
> it would be generally safer to mark the actual value as volatile inside the
> structure definition itself? In any cases where we do want to store the value
> locally and not re-access the structure, a local variable can be used.
> 
> Regards,
> /Bruce

Linux generally discourages volatile as a general style guidance:
https://www.kernel.org/doc/Documentation/volatile-considered-harmful.txt
it doesn't have to apply to dpdk which has a different coding style
but IIUC this structure is inherited from linux, deviating
will make keeping things up to date harder.

> > 
> > It might be a good idea to wrap this in a macro
> > similar to ACCESS_ONCE in Linux.
> > 
> > > 
> > > Stephen:
> > > Another question is why we need a loop here?
> > > 
> > > /huawei
> > 
> > -- 
> > MST