From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bruce Richardson Subject: Re: [PATCH] virtio: use volatile to get used->idx in the loop Date: Wed, 25 May 2016 11:00:39 +0100 Message-ID: <20160525100039.GA7228@bricha3-MOBL3> References: <1464106601-981-1-git-send-email-huawei.xie@intel.com> <20160525113224-mutt-send-email-mst@redhat.com> <20160525094729.GA4256@bricha3-MOBL3> <20160525124815-mutt-send-email-mst@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Xie, Huawei" , "dev@dpdk.org" , "stephen@networkplumber.org" , "Ananyev, Konstantin" , "thomas.monjalon@6wind.com" , Yuanhan Liu , "Tan, Jianfeng" To: "Michael S. Tsirkin" Return-path: Received: from mga04.intel.com (mga04.intel.com [192.55.52.120]) by dpdk.org (Postfix) with ESMTP id 1A8D55677 for ; Wed, 25 May 2016 12:00:45 +0200 (CEST) Content-Disposition: inline In-Reply-To: <20160525124815-mutt-send-email-mst@redhat.com> List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" On Wed, May 25, 2016 at 12:50:02PM +0300, Michael S. Tsirkin wrote: > 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 > > > > > --- > > > > > 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. The prohibition on volatile indeed doesn't apply to DPDK, due to the fact that we so seldom use locks, and do a lot of direct register accesses in out PMDs. [I also still have the scars from previous issues where we had nice subtle bugs in our PMDs - which only occurred with specific subversions of gcc - all due to a missing "volatile" on one structure element.] However, in this case, I take your point about keeping things consistent with the kernel. :-) /Bruce > > > > > > > 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