[PATCH v2 2/3] virtio-balloon: Add locking to prevent possible race when starting hinting

[Alexander Duyck <alexander.duyck@gmail.com>]

From: Alexander Duyck <alexander.h.duyck@linux.intel.com>

There is already locking in place when we are stopping free page hinting
but there is not similar protections in place when we start. I can only
assume this was overlooked as in most cases the page hinting should not be
occurring when we are starting the hinting, however there is still a chance
we could be processing hints by the time we get back around to restarting
the hinting so we are better off making sure to protect the state with the
mutex lock rather than just updating the value with no protections.

Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
---
 hw/virtio/virtio-balloon.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index 0c0fd7114799..b3e96a822b4d 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -593,6 +593,8 @@ static void virtio_balloon_free_page_start(VirtIOBalloon *s)
         return;
     }
 
+    qemu_mutex_lock(&s->free_page_lock);
+
     if (s->free_page_report_cmd_id == UINT_MAX) {
         s->free_page_report_cmd_id =
                        VIRTIO_BALLOON_FREE_PAGE_REPORT_CMD_ID_MIN;
@@ -601,6 +603,8 @@ static void virtio_balloon_free_page_start(VirtIOBalloon *s)
     }
 
     s->free_page_report_status = FREE_PAGE_REPORT_S_REQUESTED;
+    qemu_mutex_unlock(&s->free_page_lock);
+
     virtio_notify_config(vdev);
 }