ublk: add UBLK_CMD_TRY_STOP_DEV command

This command is similar to UBLK_CMD_STOP_DEV, but it only stops the
device if there are no active openers for the ublk block device.
If the device is busy, the command returns -EBUSY instead of
disrupting active clients. This allows safe, non-destructive stopping.

Signed-off-by: Yoav Cohen <[email protected]>

Author: yoavcoh

drivers/block/ublk_drv.c

@@ -54,6 +54,7 @@
 #define UBLK_CMD_DEL_DEV_ASYNC	_IOC_NR(UBLK_U_CMD_DEL_DEV_ASYNC)
 #define UBLK_CMD_UPDATE_SIZE	_IOC_NR(UBLK_U_CMD_UPDATE_SIZE)
 #define UBLK_CMD_QUIESCE_DEV	_IOC_NR(UBLK_U_CMD_QUIESCE_DEV)
+#define UBLK_CMD_TRY_STOP_DEV	_IOC_NR(UBLK_U_CMD_TRY_STOP_DEV)
 
 #define UBLK_IO_REGISTER_IO_BUF		_IOC_NR(UBLK_U_IO_REGISTER_IO_BUF)
 #define UBLK_IO_UNREGISTER_IO_BUF	_IOC_NR(UBLK_U_IO_UNREGISTER_IO_BUF)
@@ -238,6 +239,8 @@ struct ublk_device {
 	pid_t 	ublksrv_tgid;
 	struct delayed_work	exit_work;
 
+	bool			block_open; /* protected by open_mutex */
+
 	struct ublk_queue       *queues[];
 };
 
@@ -3280,6 +3283,38 @@ static void ublk_ctrl_stop_dev(struct ublk_device *ub)
 	ublk_stop_dev(ub);
 }
 
+static int ublk_ctrl_try_stop_dev(struct ublk_device *ub)
+{
+	struct gendisk *disk;
+	int ret = -EINVAL;
+
+	disk = ublk_get_disk(ub);
+	if (!disk) {
+		ret = -ENODEV;
+		goto out;
+	}
+
+	mutex_lock(&disk->open_mutex);
+	if (disk_openers(disk) > 0) {
+		ret = -EBUSY;
+		goto unlock;
+	}
+	ub->block_open = true;
+	/* release open_mutex as del_gendisk() will reacquire it */
+	mutex_unlock(&disk->open_mutex);
+
+	ublk_ctrl_stop_dev(ub);
+	ret = 0;
+	goto put_disk;
+
+unlock:
+	mutex_unlock(&disk->open_mutex);
+put_disk:
+	ublk_put_disk(disk);
+out:
+	return ret;
+}
+
 static int ublk_ctrl_get_dev_info(struct ublk_device *ub,
 		const struct ublksrv_ctrl_cmd *header)
 {
@@ -3675,6 +3710,7 @@ static int ublk_ctrl_uring_cmd_permission(struct ublk_device *ub,
 	case UBLK_CMD_END_USER_RECOVERY:
 	case UBLK_CMD_UPDATE_SIZE:
 	case UBLK_CMD_QUIESCE_DEV:
+	case UBLK_CMD_TRY_STOP_DEV:
 		mask = MAY_READ | MAY_WRITE;
 		break;
 	default:
@@ -3788,6 +3824,9 @@ static int ublk_ctrl_uring_cmd(struct io_uring_cmd *cmd,
 	case UBLK_CMD_QUIESCE_DEV:
 		ret = ublk_ctrl_quiesce_dev(ub, header);
 		break;
+	case UBLK_CMD_TRY_STOP_DEV:
+		ret = ublk_ctrl_try_stop_dev(ub);
+		break;
 	default:
 		ret = -EOPNOTSUPP;
 		break;

include/uapi/linux/ublk_cmd.h

@@ -55,7 +55,8 @@
 	_IOWR('u', 0x15, struct ublksrv_ctrl_cmd)
 #define UBLK_U_CMD_QUIESCE_DEV		\
 	_IOWR('u', 0x16, struct ublksrv_ctrl_cmd)
-
+#define UBLK_U_CMD_TRY_STOP_DEV		\
+	_IOWR('u', 0x17, struct ublksrv_ctrl_cmd)
 /*
  * 64bits are enough now, and it should be easy to extend in case of
  * running out of feature flags