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.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.
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
