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

[Wei Wang <wei.w.wang@intel.com>]

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. 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