drm/syncobj: Stop reusing the same struct file for all syncobj -> fd

The vk cts test:
dEQP-VK.api.external.semaphore.opaque_fd.export_multiple_times_temporary

triggers a lot of
VFS: Close: file count is 0

Dave pointed out that clearing the syncobj->file from
drm_syncobj_file_release() was sufficient to silence the test, but that
opens a can of worm since we assumed that the syncobj->file was never
unset. Stop trying to reuse the same struct file for every fd pointing
to the drm_syncobj, and allocate one file for each fd instead.

v2: Fixup return handling of drm_syncobj_fd_to_handle
v2.1: [airlied: fix possible syncobj ref race]

Reported-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_syncobj.c

@@ -369,40 +369,26 @@ static const struct file_operations drm_syncobj_file_fops = {
 	.release = drm_syncobj_file_release,
 };
 
-static int drm_syncobj_alloc_file(struct drm_syncobj *syncobj)
-{
-	struct file *file = anon_inode_getfile("syncobj_file",
-					       &drm_syncobj_file_fops,
-					       syncobj, 0);
-	if (IS_ERR(file))
-		return PTR_ERR(file);
-
-	drm_syncobj_get(syncobj);
-	if (cmpxchg(&syncobj->file, NULL, file)) {
-		/* lost the race */
-		fput(file);
-	}
-
-	return 0;
-}
-
 int drm_syncobj_get_fd(struct drm_syncobj *syncobj, int *p_fd)
 {
-	int ret;
+	struct file *file;
 	int fd;
 
 	fd = get_unused_fd_flags(O_CLOEXEC);
 	if (fd < 0)
 		return fd;
 
-	if (!syncobj->file) {
-		ret = drm_syncobj_alloc_file(syncobj);
-		if (ret) {
-			put_unused_fd(fd);
-			return ret;
-		}
+	file = anon_inode_getfile("syncobj_file",
+				  &drm_syncobj_file_fops,
+				  syncobj, 0);
+	if (IS_ERR(file)) {
+		put_unused_fd(fd);
+		return PTR_ERR(file);
 	}
-	fd_install(fd, syncobj->file);
+
+	drm_syncobj_get(syncobj);
+	fd_install(fd, file);
+
 	*p_fd = fd;
 	return 0;
 }
@@ -422,31 +408,24 @@ static int drm_syncobj_handle_to_fd(struct drm_file *file_private,
 	return ret;
 }
 
-static struct drm_syncobj *drm_syncobj_fdget(int fd)
-{
-	struct file *file = fget(fd);
-
-	if (!file)
-		return NULL;
-	if (file->f_op != &drm_syncobj_file_fops)
-		goto err;
-
-	return file->private_data;
-err:
-	fput(file);
-	return NULL;
-};
-
 static int drm_syncobj_fd_to_handle(struct drm_file *file_private,
 				    int fd, u32 *handle)
 {
-	struct drm_syncobj *syncobj = drm_syncobj_fdget(fd);
+	struct drm_syncobj *syncobj;
+	struct file *file;
 	int ret;
 
-	if (!syncobj)
+	file = fget(fd);
+	if (!file)
 		return -EINVAL;
 
+	if (file->f_op != &drm_syncobj_file_fops) {
+		fput(file);
+		return -EINVAL;
+	}
+
 	/* take a reference to put in the idr */
+	syncobj = file->private_data;
 	drm_syncobj_get(syncobj);
 
 	idr_preload(GFP_KERNEL);
@@ -455,12 +434,14 @@ static int drm_syncobj_fd_to_handle(struct drm_file *file_private,
 	spin_unlock(&file_private->syncobj_table_lock);
 	idr_preload_end();
 
-	if (ret < 0) {
-		fput(syncobj->file);
-		return ret;
-	}
-	*handle = ret;
-	return 0;
+	if (ret > 0) {
+		*handle = ret;
+		ret = 0;
+	} else
+		drm_syncobj_put(syncobj);
+
+	fput(file);
+	return ret;
 }
 
 static int drm_syncobj_import_sync_file_fence(struct drm_file *file_private,