ublk: add UBLK_CMD_TRY_STOP_DEV command

Add a best-effort stop command, UBLK_CMD_TRY_STOP_DEV, which only stops a
ublk device when it has no active openers.

Unlike UBLK_CMD_STOP_DEV, this command does not disrupt existing users.
New opens are blocked only after disk_openers has reached zero; if the
device is busy, the command returns -EBUSY and leaves it running.

The ub->block_open flag is used only to close a race with an in-progress
open and does not otherwise change open behavior.

Advertise support via the UBLK_F_SAFE_STOP_DEV feature flag.

Signed-off-by: Yoav Cohen <yoav@nvidia.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: yoavcoh

drivers/block/ublk_drv.c

@@ -56,6 +56,7 @@
 #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)
@@ -76,7 +77,8 @@
 		| UBLK_F_QUIESCE \
 		| UBLK_F_PER_IO_DAEMON \
 		| UBLK_F_BUF_REG_OFF_DAEMON \
-		| (IS_ENABLED(CONFIG_BLK_DEV_INTEGRITY) ? UBLK_F_INTEGRITY : 0))
+		| (IS_ENABLED(CONFIG_BLK_DEV_INTEGRITY) ? UBLK_F_INTEGRITY : 0) \
+		| UBLK_F_SAFE_STOP_DEV)
 
 #define UBLK_F_ALL_RECOVERY_FLAGS (UBLK_F_USER_RECOVERY \
 		| UBLK_F_USER_RECOVERY_REISSUE \
@@ -243,6 +245,8 @@ struct ublk_device {
 	struct delayed_work	exit_work;
 	struct work_struct	partition_scan_work;
 
+	bool			block_open; /* protected by open_mutex */
+
 	struct ublk_queue       *queues[];
 };
 
@@ -984,6 +988,9 @@ static int ublk_open(struct gendisk *disk, blk_mode_t mode)
 			return -EPERM;
 	}
 
+	if (ub->block_open)
+		return -ENXIO;
+
 	return 0;
 }
 
@@ -3343,7 +3350,8 @@ static int ublk_ctrl_add_dev(const struct ublksrv_ctrl_cmd *header)
 	ub->dev_info.flags |= UBLK_F_CMD_IOCTL_ENCODE |
 		UBLK_F_URING_CMD_COMP_IN_TASK |
 		UBLK_F_PER_IO_DAEMON |
-		UBLK_F_BUF_REG_OFF_DAEMON;
+		UBLK_F_BUF_REG_OFF_DAEMON |
+		UBLK_F_SAFE_STOP_DEV;
 
 	/* GET_DATA isn't needed any more with USER_COPY or ZERO COPY */
 	if (ub->dev_info.flags & (UBLK_F_USER_COPY | UBLK_F_SUPPORT_ZERO_COPY |
@@ -3464,6 +3472,34 @@ 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 = 0;
+
+	disk = ublk_get_disk(ub);
+	if (!disk)
+		return -ENODEV;
+
+	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);
+	goto out;
+
+unlock:
+	mutex_unlock(&disk->open_mutex);
+out:
+	ublk_put_disk(disk);
+	return ret;
+}
+
 static int ublk_ctrl_get_dev_info(struct ublk_device *ub,
 		const struct ublksrv_ctrl_cmd *header)
 {
@@ -3859,6 +3895,7 @@ 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:
@@ -3972,6 +4009,9 @@ 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
@@ -321,6 +322,12 @@
  */
 #define UBLK_F_INTEGRITY (1ULL << 16)
 
+/*
+ * The device supports the UBLK_CMD_TRY_STOP_DEV command, which
+ * allows stopping the device only if there are no openers.
+ */
+#define UBLK_F_SAFE_STOP_DEV	(1ULL << 17)
+
 /* device state */
 #define UBLK_S_DEV_DEAD	0
 #define UBLK_S_DEV_LIVE	1