[PATCH] drm/virtio: fix virtio-gpu resource id creation race

[John Bates]

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.

Signed-off-by: John Bates <jbates at chromium.org>
---

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

diff --git a/drivers/gpu/drm/virtio/virtgpu_object.c b/drivers/gpu/drm/virtio/virtgpu_object.c
index 017a9e0fc3bb..b11c1fce1770 100644
--- 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(struct virtio_gpu_device *vgdev,
 		 * "f91a9dd35715 Fix unlinking resources from hash
 		 * table." (Feb 2019) fixes the bug.
 		 */
-		static int handle;
-		handle++;
+		static atomic_t seqno;
+		int handle = atomic_inc_return(&seqno);
 		*resid = handle + 1;
 	} else {
 		int handle = ida_alloc(&vgdev->resource_ida, GFP_KERNEL);
-- 
2.25.0.265.gbab2e86ba0-goog