Re: [virtio-dev] Re: [PATCH v4 3/4] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

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

On Wed, Mar 14, 2018 at 02:03:19PM +0800, Wei Wang wrote:
> On 03/14/2018 10:53 AM, Michael S. Tsirkin wrote:
> > On Wed, Mar 14, 2018 at 10:43:01AM +0800, Wei Wang wrote:
> > > On 03/14/2018 12:49 AM, Michael S. Tsirkin wrote:
> > > > On Wed, Mar 07, 2018 at 08:34:24PM +0800, Wei Wang wrote:
> > > > 
> > > > > Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> > > > > Signed-off-by: Liang Li <liang.z.li@intel.com>
> > > > > CC: Michael S. Tsirkin <mst@redhat.com>
> > > > > CC: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > > > CC: Juan Quintela <quintela@redhat.com>
> > > > I find it suspicious that neither unrealize nor reset
> > > > functions have been touched at all.
> > > > Are you sure you have thought through scenarious like
> > > > hot-unplug or disabling the device by guest?
> > > OK. I think we can call balloon_free_page_stop in unrealize and reset.
> > > 
> > > 
> > > > +static void *virtio_balloon_poll_free_page_hints(void *opaque)
> > > > +{
> > > > +    VirtQueueElement *elem;
> > > > +    VirtIOBalloon *dev = opaque;
> > > > +    VirtQueue *vq = dev->free_page_vq;
> > > > +    uint32_t id;
> > > > +    size_t size;
> > > > What makes it safe to poke at this device from multiple threads?
> > > > I think that it would be safer to do it from e.g. BH.
> > > > 
> > > Actually the free_page_optimization thread is the only user of free_page_vq,
> > > and there is only one optimization thread each time. Would this be safe
> > > enough?
> > > 
> > > Best,
> > > Wei
> > Aren't there other fields there? Also things like reset affect all VQs.
> > 
> 
> Yes. But I think BHs are used to avoid re-entrancy, which isn't the issue
> here.

Since you are adding locks to address the issue - doesn't this imply
reentrancy is exactly the issue?

> The potential issue I could observe here is that
> "dev->free_page_report_status" is read and written by the optimization
> thread, and it is also modified by the migration thread and reset via
> virtio_balloon_free_page_stop.
> 
> How about adding a QEMU SpinLock, like this:
> 
> virtio_balloon_poll_free_page_hints()
> {
> 
>     while (1) {
>         qemu_spin_lock();
>         /* If the status has been changed to STOP or EXIT, or the VM is
> stopped, just exit */
>         if (dev->free_page_report_status >= FREE_PAGE_REPORT_S_STOP ||
> !runstate_is_running()) {
>             qemu_spin_unlock();
>             break;
>         }
>         ....
>         qemu_spin_unlock();
>     }
> }
> 
> 
> Best,
> Wei

That will address the issue but it does look weird.

-- 
MST