virtio-balloon: Add locking to prevent possible race when starting hinting

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.

Based on feedback from Peter Maydell this issue had also been spotted by
Coverity: CID 1430269

Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Message-Id: <20200720175122.21935.78013.stgit@localhost.localdomain>
Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 1a83e0b9c492a0eaeacd6fbb858fc81d04ab9c3e)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index a30a0c7bfa1294aad2a42e35591c391eff0fb633..d6e31de1d9a80a60b6d46882f6c33a86c6399ec8 100644 (file)
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -532,6 +532,8 @@ 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;
@@ -540,6 +542,8 @@ 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);
 }