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.w...@intel.com>
Signed-off-by: Liang Li <liang.z...@intel.com>
CC: Michael S. Tsirkin <m...@redhat.com>
CC: Dr. David Alan Gilbert <dgilb...@redhat.com>
CC: Juan Quintela <quint...@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.

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

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