virgl: Avoid unfinished transfer_get with PIPE_TRANSFER_DONTBLOCK

If we are not allowed to block, and we know that we will have to wait,
either because the resource is busy, or because it will become busy due
to a readback, return early to avoid performing an incomplete
transfer_get. Such an incomplete transfer_get may finish at any time,
during which another unsynchronized map could write to the resource
contents, leaving the contents in an undefined state.

Signed-off-by: Alexandros Frantzis <[email protected]>
Suggested-by: Chia-I Wu <[email protected]>
Reviewed-by: Chia-I Wu <[email protected]>

1 files changed, 12 insertions, 9 deletions

diff --git a/src/gallium/drivers/virgl/virgl_resource.c b/src/gallium/drivers/virgl/virgl_resource.c
index 8e3d6b38423..1cb9feb3658 100644
--- a/src/gallium/drivers/virgl/virgl_resource.c
+++ b/src/gallium/drivers/virgl/virgl_resource.c
@@ -147,21 +147,24 @@ virgl_resource_transfer_prepare(struct virgl_context *vctx,
    if (flush)
       vctx->base.flush(&vctx->base, NULL, 0);
 
+   /* If we are not allowed to block, and we know that we will have to wait,
+    * either because the resource is busy, or because it will become busy due
+    * to a readback, return early to avoid performing an incomplete
+    * transfer_get. Such an incomplete transfer_get may finish at any time,
+    * during which another unsynchronized map could write to the resource
+    * contents, leaving the contents in an undefined state.
+    */
+   if ((xfer->base.usage & PIPE_TRANSFER_DONTBLOCK) &&
+       (readback || (wait && vws->resource_is_busy(vws, res->hw_res))))
+      return VIRGL_TRANSFER_MAP_ERROR;
+
    if (readback) {
       vws->transfer_get(vws, res->hw_res, &xfer->base.box, xfer->base.stride,
                         xfer->l_stride, xfer->offset, xfer->base.level);
    }
 
-   if (wait) {
-      /* fail the mapping after flush and readback so that it will succeed in
-       * the future
-       */
-      if ((xfer->base.usage & PIPE_TRANSFER_DONTBLOCK) &&
-          vws->resource_is_busy(vws, res->hw_res))
-         return VIRGL_TRANSFER_MAP_ERROR;
-
+   if (wait)
       vws->resource_wait(vws, res->hw_res);
-   }
 
    return map_type;
 }