drm/prime: proper locking+refcounting for obj->dma_buf link

The export dma-buf cache is semantically similar to an flink name. So
semantically it makes sense to treat it the same and remove the name
(i.e. the dma_buf pointer) and its references when the last gem handle
disappears.

Again we need to be careful, but double so: Not just could someone
race and export with a gem close ioctl (so we need to recheck
obj->handle_count again when assigning the new name), but multiple
exports can also race against each another. This is prevented by
holding the dev->object_name_lock across the entire section which
touches obj->dma_buf.

With the new scheme we also need to reinstate the obj->dma_buf link at
import time (in case the only reference userspace has held in-between
was through the dma-buf fd and not through any native gem handle). For
simplicity we don't check whether it's a native object but
unconditionally set up that link - with the new scheme of removing the
obj->dma_buf reference when the last handle disappears we can do that.

To make it clear that this is not just for exported buffers anymore
als rename it from export_dma_buf to dma_buf.

To make sure that now one can race a fd_to_handle or handle_to_fd with
gem_close we use the same tricks as in flink of extending the
dev->object_name_locking critical section. With this change we finally
have a guaranteed 1:1 relationship (at least for native objects)
between gem objects and dma-bufs, even accounting for races (which can
happen since the dma-buf itself holds a reference while in-flight).

This prevent igt/prime_self_import/export-vs-gem_close-race from
Oopsing the kernel. There is still a leak though since the per-file
priv dma-buf/handle cache handling is racy. That will be fixed in a
later patch.

v2: Remove the bogus dma_buf_put from the export_and_register_object
failure path if we've raced with the handle count dropping to 0.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

include/drm/drmP.h

@@ -667,8 +667,16 @@ struct drm_gem_object {
 
 	void *driver_private;
 
-	/* dma buf exported from this GEM object */
-	struct dma_buf *export_dma_buf;
+	/**
+	 * dma_buf - dma buf associated with this GEM object
+	 *
+	 * Pointer to the dma-buf associated with this gem object (either
+	 * through importing or exporting). We break the resulting reference
+	 * loop when the last gem handle for this object is released.
+	 *
+	 * Protected by obj->object_name_lock
+	 */
+	struct dma_buf *dma_buf;
 
 	/**
 	 * import_attach - dma buf attachment backing this object