[qemu-xen staging] virtio-balloon: Add locking to prevent possible race when starting hinting

[patchbot@xxxxxxx]

commit 1a83e0b9c492a0eaeacd6fbb858fc81d04ab9c3e
Author:     Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
AuthorDate: Mon Jul 20 10:51:22 2020 -0700
Commit:     Michael S. Tsirkin <mst@xxxxxxxxxx>
CommitDate: Wed Jul 22 07:57:07 2020 -0400

    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@xxxxxxxxxx>
    Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
    Message-Id: <20200720175122.21935.78013.stgit@localhost.localdomain>
    Cc: qemu-stable@xxxxxxxxxx
    Reviewed-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
    Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---
 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 ce70adcc69..6e2d129340 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -592,6 +592,8 @@ 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;
@@ -600,6 +602,8 @@ 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);
 }
 
--
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