[PATCH 5.5 129/189] drm/virtio: fix resource id creation race

[Greg Kroah-Hartman]

From: John Bates <jbates@xxxxxxxxxxxx>

commit fbb30168c7395b9cfeb9e6f7b0c0bca854a6552d upstream.

The previous code was not thread safe and caused
undefined behavior from spurious duplicate resource IDs.
In this patch, an atomic_t is used instead. We no longer
see any duplicate IDs in tests with this change.

Fixes: 16065fcdd19d ("drm/virtio: do NOT reuse resource ids")
Signed-off-by: John Bates <jbates@xxxxxxxxxxxx>
Reviewed-by: Chia-I Wu <olvaffe@xxxxxxxxx>
Link: http://patchwork.freedesktop.org/patch/msgid/20200220225319.45621-1-jbates@xxxxxxxxxxxx
Signed-off-by: Gerd Hoffmann <kraxel@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/gpu/drm/virtio/virtgpu_object.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/gpu/drm/virtio/virtgpu_object.c
+++ b/drivers/gpu/drm/virtio/virtgpu_object.c
@@ -42,8 +42,8 @@ static int virtio_gpu_resource_id_get(st
 		 * "f91a9dd35715 Fix unlinking resources from hash
 		 * table." (Feb 2019) fixes the bug.
 		 */
-		static int handle;
-		handle++;
+		static atomic_t seqno = ATOMIC_INIT(0);
+		int handle = atomic_inc_return(&seqno);
 		*resid = handle + 1;
 	} else {
 		int handle = ida_alloc(&vgdev->resource_ida, GFP_KERNEL);