Re: [virtio-dev] Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

[Michael S. Tsirkin]

On Wed, Jan 24, 2018 at 11:18:40AM +0800, Wei Wang wrote:
> On 01/22/2018 07:25 PM, Wei Wang wrote:
> > On 01/19/2018 08:39 PM, Michael S. Tsirkin wrote:
> > > On Fri, Jan 19, 2018 at 11:44:21AM +0800, Wei Wang wrote:
> > > > On 01/18/2018 12:44 AM, Michael S. Tsirkin wrote:
> > > > > On Wed, Jan 17, 2018 at 01:10:11PM +0800, Wei Wang wrote:
> > > > > 
> > > > > > +        vb->start_cmd_id = cmd_id;
> > > > > > +        queue_work(vb->balloon_wq, &vb->report_free_page_work);
> > > > > It seems that if a command was already queued (with a different id),
> > > > > this will result in new command id being sent to host twice,
> > > > > which will
> > > > > likely confuse the host.
> > > > I think that case won't happen, because
> > > > - the host sends a cmd id to the guest via the config, while the
> > > > guest acks
> > > > back the received cmd id via the virtqueue;
> > > > - the guest ack back a cmd id only when a new cmd id is received
> > > > from the
> > > > host, that is the above check:
> > > > 
> > > >      if (cmd_id != vb->start_cmd_id) { --> the driver only queues the
> > > > reporting work only when a new cmd id is received
> > > >                          /*
> > > >                           * Host requests to start the reporting
> > > > by sending a
> > > >                           * new cmd id.
> > > >                           */
> > > >                          WRITE_ONCE(vb->report_free_page, true);
> > > >                          vb->start_cmd_id = cmd_id;
> > > >                          queue_work(vb->balloon_wq,
> > > > &vb->report_free_page_work);
> > > >      }
> > > > 
> > > > So the same cmd id wouldn't queue the reporting work twice.
> > > > 
> > > Like this:
> > > 
> > >         vb->start_cmd_id = cmd_id;
> > >         queue_work(vb->balloon_wq, &vb->report_free_page_work);
> > > 
> > > command id changes
> > > 
> > >         vb->start_cmd_id = cmd_id;
> > > 
> > > work executes
> > > 
> > >         queue_work(vb->balloon_wq, &vb->report_free_page_work);
> > > 
> > > work executes again
> > > 
> > 
> > If we think about the whole working flow, I think this case couldn't
> > happen:
> > 
> > 1) device send cmd_id=1 to driver;
> > 2) driver receives cmd_id=1 in the config and acks cmd_id=1 to the
> > device via the vq;
> > 3) device revives cmd_id=1;
> > 4) device wants to stop the reporting by sending cmd_id=STOP;
> > 5) driver receives cmd_id=STOP from the config, and acks cmd_id=STOP to
> > the device via the vq;
> > 6) device sends cmd_id=2 to driver;
> > ...
> > 
> > cmd_id=2 won't come after cmd_id=1, there will be a STOP cmd in between
> > them (STOP won't queue the work).
> > 
> > How about defining the correct device behavior in the spec:
> > The device Should NOT send a second cmd id to the driver until a STOP
> > cmd ack for the previous cmd id has been received from the guest.
> 
> 
> Thanks for the comments, and I adopted most of them in the new posted v23
> patches. The above discussion is the one that I haven't included. If you
> could still see issues in the above analysis, please let me know. Thanks.
> 
> Best,
> Wei
> 
> 
>

Yes, I think you should just fix the race in the driver.

-- 
MST