selftests: ublk: kublk: improve behavior on init failure

Some failure modes are handled poorly by kublk. For example, if ublk_drv
is built as a module but not currently loaded into the kernel, ./kublk
add ... just hangs forever. This happens because in this case (and a few
others), the worker process does not notify its parent (via a write to
the shared eventfd) that it has tried and failed to initialize, so the
parent hangs forever. Fix this by ensuring that we always notify the
parent process of any initialization failure, and have the parent print
a (not very descriptive) log line when this happens.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250603-ublk_init_fail-v1-1-87c91486230e@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: ps-ushankar

tools/testing/selftests/ublk/kublk.c

@@ -1112,21 +1112,23 @@ static int __cmd_dev_add(const struct dev_ctx *ctx)
 	__u64 features;
 	const struct ublk_tgt_ops *ops;
 	struct ublksrv_ctrl_dev_info *info;
-	struct ublk_dev *dev;
+	struct ublk_dev *dev = NULL;
 	int dev_id = ctx->dev_id;
 	int ret, i;
 
 	ops = ublk_find_tgt(tgt_type);
 	if (!ops) {
 		ublk_err("%s: no such tgt type, type %s\n",
 				__func__, tgt_type);
-		return -ENODEV;
+		ret = -ENODEV;
+		goto fail;
 	}
 
 	if (nr_queues > UBLK_MAX_QUEUES || depth > UBLK_QUEUE_DEPTH) {
 		ublk_err("%s: invalid nr_queues or depth queues %u depth %u\n",
 				__func__, nr_queues, depth);
-		return -EINVAL;
+		ret = -EINVAL;
+		goto fail;
 	}
 
 	/* default to 1:1 threads:queues if nthreads is unspecified */
@@ -1136,30 +1138,37 @@ static int __cmd_dev_add(const struct dev_ctx *ctx)
 	if (nthreads > UBLK_MAX_THREADS) {
 		ublk_err("%s: %u is too many threads (max %u)\n",
 				__func__, nthreads, UBLK_MAX_THREADS);
-		return -EINVAL;
+		ret = -EINVAL;
+		goto fail;
 	}
 
 	if (nthreads != nr_queues && !ctx->per_io_tasks) {
 		ublk_err("%s: threads %u must be same as queues %u if "
 			"not using per_io_tasks\n",
 			__func__, nthreads, nr_queues);
-		return -EINVAL;
+		ret = -EINVAL;
+		goto fail;
 	}
 
 	dev = ublk_ctrl_init();
 	if (!dev) {
 		ublk_err("%s: can't alloc dev id %d, type %s\n",
 				__func__, dev_id, tgt_type);
-		return -ENOMEM;
+		ret = -ENOMEM;
+		goto fail;
 	}
 
 	/* kernel doesn't support get_features */
 	ret = ublk_ctrl_get_features(dev, &features);
-	if (ret < 0)
-		return -EINVAL;
+	if (ret < 0) {
+		ret = -EINVAL;
+		goto fail;
+	}
 
-	if (!(features & UBLK_F_CMD_IOCTL_ENCODE))
-		return -ENOTSUP;
+	if (!(features & UBLK_F_CMD_IOCTL_ENCODE)) {
+		ret = -ENOTSUP;
+		goto fail;
+	}
 
 	info = &dev->dev_info;
 	info->dev_id = ctx->dev_id;
@@ -1200,7 +1209,8 @@ static int __cmd_dev_add(const struct dev_ctx *ctx)
 fail:
 	if (ret < 0)
 		ublk_send_dev_event(ctx, dev, -1);
-	ublk_ctrl_deinit(dev);
+	if (dev)
+		ublk_ctrl_deinit(dev);
 	return ret;
 }
 
@@ -1262,6 +1272,8 @@ static int cmd_dev_add(struct dev_ctx *ctx)
 		shmctl(ctx->_shmid, IPC_RMID, NULL);
 		/* wait for child and detach from it */
 		wait(NULL);
+		if (exit_code == EXIT_FAILURE)
+			ublk_err("%s: command failed\n", __func__);
 		exit(exit_code);
 	} else {
 		exit(EXIT_FAILURE);