drm/virtio: fix resource id creation race

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@chromium.org>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200220225319.45621-1-jbates@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/gpu/drm/virtio/virtgpu_object.c b/drivers/gpu/drm/virtio/virtgpu_object.c
index aab5534..3e538f8 100644 (file)
--- 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 = ATOMIC_INIT(0);
+               int handle = atomic_inc_return(&seqno);
                *resid = handle + 1;
        } else {
                int handle = ida_alloc(&vgdev->resource_ida, GFP_KERNEL);