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

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

On Thu, Mar 15, 2018 at 09:15:48AM +0800, Wei Wang wrote:
> On 03/14/2018 10:12 PM, Michael S. Tsirkin wrote:
> > 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?
> 
> Not really. The lock isn't intended for any reentrancy issues, since there
> will be only one run of the virtio_balloon_poll_free_page_hints function at
> any given time. Instead, the lock is used to synchronize
> virtio_balloon_poll_free_page_hints and virtio_balloon_free_page_stop to
> access dev->free_page_report_status.

I wonder whether that's enough. E.g. is there a race with guest
trying to reset the device? That resets all VQs you know.


> Please see the whole picture below:
> 
> virtio_balloon_poll_free_page_hints()
> {
> 
>     while (1) {
>         qemu_spin_lock();
>         if (dev->free_page_report_status >= FREE_PAGE_REPORT_S_STOP ||
>             !runstate_is_running()) {
>             qemu_spin_unlock();
>             break;
>         }
>         ...
>         if (id == dev->free_page_report_cmd_id) {
> ==>        dev->free_page_report_status = FREE_PAGE_REPORT_S_START;
>         ...
>         qemu_spin_unlock();
>     }
> }
> 
> 
> static void virtio_balloon_free_page_stop(void *opaque)
> {
>     VirtIOBalloon *s = opaque;
>     VirtIODevice *vdev = VIRTIO_DEVICE(s);
> 
>     qemu_spin_lock();
> ...
> ==>       s->free_page_report_status = FREE_PAGE_REPORT_S_STOP;
>     ...
>     qemu_spin_unlock();
> }
> 
> 
> Without the lock, there are theoretical possibilities that assigning STOP
> below is overridden by START above. In that
> case,virtio_balloon_free_page_stop does not effectively stop
> virtio_balloon_poll_free_page_hints.
> I think this issue couldn't be solved by BHs.
> 
> Best,
> Wei

Don't all BHs run under the BQL?