block: allow gendisk's request_queue registration to be deferred

Since I can remember DM has forced the block layer to allow the
allocation and initialization of the request_queue to be distinct
operations.  Reason for this is block/genhd.c:add_disk() has requires
that the request_queue (and associated bdi) be tied to the gendisk
before add_disk() is called -- because add_disk() also deals with
exposing the request_queue via blk_register_queue().

DM's dynamic creation of arbitrary device types (and associated
request_queue types) requires the DM device's gendisk be available so
that DM table loads can establish a master/slave relationship with
subordinate devices that are referenced by loaded DM tables -- using
bd_link_disk_holder().  But until these DM tables, and their associated
subordinate devices, are known DM cannot know what type of request_queue
it needs -- nor what its queue_limits should be.

This chicken and egg scenario has created all manner of problems for DM
and, at times, the block layer.

Summary of changes:

- Add device_add_disk_no_queue_reg() and add_disk_no_queue_reg() variant
  that drivers may use to add a disk without also calling
  blk_register_queue().  Driver must call blk_register_queue() once its
  request_queue is fully initialized.

- Return early from blk_unregister_queue() if QUEUE_FLAG_REGISTERED
  is not set.  It won't be set if driver used add_disk_no_queue_reg()
  but driver encounters an error and must del_gendisk() before calling
  blk_register_queue().

- Export blk_register_queue().

These changes allow DM to use add_disk_no_queue_reg() to anchor its
gendisk as the "master" for master/slave relationships DM must establish
with subordinate devices referenced in DM tables that get loaded.  Once
all "slave" devices for a DM device are known its request_queue can be
properly initialized and then advertised via sysfs -- important
improvement being that no request_queue resource initialization
performed by blk_register_queue() is missed for DM devices anymore.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-sysfs.c

@@ -921,6 +921,7 @@ unlock:
 	mutex_unlock(&q->sysfs_lock);
 	return ret;
 }
+EXPORT_SYMBOL_GPL(blk_register_queue);
 
 void blk_unregister_queue(struct gendisk *disk)
 {
@@ -929,6 +930,10 @@ void blk_unregister_queue(struct gendisk *disk)
 	if (WARN_ON(!q))
 		return;
 
+	/* Return early if disk->queue was never registered. */
+	if (!test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags))
+		return;
+
 	/*
 	 * Protect against the 'queue' kobj being accessed
 	 * while/after it is removed.

block/genhd.c

@@ -629,16 +629,18 @@ exit:
 }
 
 /**
- * device_add_disk - add partitioning information to kernel list
+ * __device_add_disk - add disk information to kernel list
  * @parent: parent device for the disk
  * @disk: per-device partitioning information
+ * @register_queue: register the queue if set to true
  *
  * This function registers the partitioning information in @disk
  * with the kernel.
  *
  * FIXME: error handling
  */
-void device_add_disk(struct device *parent, struct gendisk *disk)
+static void __device_add_disk(struct device *parent, struct gendisk *disk,
+			      bool register_queue)
 {
 	dev_t devt;
 	int retval;
@@ -682,6 +684,7 @@ void device_add_disk(struct device *parent, struct gendisk *disk)
 				    exact_match, exact_lock, disk);
 	}
 	register_disk(parent, disk);
+	if (register_queue)
 		blk_register_queue(disk);
 
 	/*
@@ -693,8 +696,19 @@ void device_add_disk(struct device *parent, struct gendisk *disk)
 	disk_add_events(disk);
 	blk_integrity_add(disk);
 }
+
+void device_add_disk(struct device *parent, struct gendisk *disk)
+{
+	__device_add_disk(parent, disk, true);
+}
 EXPORT_SYMBOL(device_add_disk);
 
+void device_add_disk_no_queue_reg(struct device *parent, struct gendisk *disk)
+{
+	__device_add_disk(parent, disk, false);
+}
+EXPORT_SYMBOL(device_add_disk_no_queue_reg);
+
 void del_gendisk(struct gendisk *disk)
 {
 	struct disk_part_iter piter;

include/linux/genhd.h

@@ -395,6 +395,11 @@ static inline void add_disk(struct gendisk *disk)
 {
 	device_add_disk(NULL, disk);
 }
+extern void device_add_disk_no_queue_reg(struct device *parent, struct gendisk *disk);
+static inline void add_disk_no_queue_reg(struct gendisk *disk)
+{
+	device_add_disk_no_queue_reg(NULL, disk);
+}
 
 extern void del_gendisk(struct gendisk *gp);
 extern struct gendisk *get_gendisk(dev_t dev, int *partno);