[for-4.16 PATCH v3 2/3] block: allow gendisk's request_queue registration to be deferred

[Mike Snitzer <snitzer@redhat.com>]

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 QUEUE_FLAG_DEFER_REG that a block driver can set to defer the
  required blk_register_queue() until the block driver's request_queue
  is fully initialized.

- Return early from blk_unregister_queue() if QUEUE_FLAG_REGISTERED
  is not set.  It won't be set if a request_queue with
  QUEUE_FLAG_DEFER_REG set never called blk_register_queue() -- as can
  happen if a driver encounters an error and must del_gendisk() before
  calling blk_register_queue().

- Export blk_register_queue().

These changes allow DM to use device_add_disk() 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 a request_queue can be
properly initialized and then advertised via sysfs -- important
improvement being that no request_queue resource initialization is
missed -- before these changes DM was known to be missing blk-mq debugfs
and proper block throttle initialization.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 block/blk-sysfs.c      | 4 ++++
 block/genhd.c          | 4 +++-
 include/linux/blkdev.h | 1 +
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 870484eaed1f..2395122875b4 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -921,6 +921,7 @@ int blk_register_queue(struct gendisk *disk)
 	mutex_unlock(&q->sysfs_lock);
 	return ret;
 }
+EXPORT_SYMBOL_GPL(blk_register_queue);
 
 void blk_unregister_queue(struct gendisk *disk)
 {
@@ -929,6 +930,9 @@ void blk_unregister_queue(struct gendisk *disk)
 	if (WARN_ON(!q))
 		return;
 
+	if (!test_bit(QUEUE_FLAG_REGISTERED, &disk->queue->queue_flags))
+		return;
+
 	mutex_lock(&q->sysfs_lock);
 	queue_flag_clear_unlocked(QUEUE_FLAG_REGISTERED, q);
 	mutex_unlock(&q->sysfs_lock);
diff --git a/block/genhd.c b/block/genhd.c
index 00620e01e043..3912a82ecc4f 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -682,7 +682,6 @@ void device_add_disk(struct device *parent, struct gendisk *disk)
 				    exact_match, exact_lock, disk);
 	}
 	register_disk(parent, disk);
-	blk_register_queue(disk);
 
 	/*
 	 * Take an extra ref on queue which will be put on disk_release()
@@ -692,6 +691,9 @@ void device_add_disk(struct device *parent, struct gendisk *disk)
 
 	disk_add_events(disk);
 	blk_integrity_add(disk);
+
+	if (!test_bit(QUEUE_FLAG_DEFER_REG, &disk->queue->queue_flags))
+		blk_register_queue(disk);
 }
 EXPORT_SYMBOL(device_add_disk);
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 71a9371c8182..84d144c7b025 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -681,6 +681,7 @@ struct request_queue {
 #define QUEUE_FLAG_SCSI_PASSTHROUGH 27	/* queue supports SCSI commands */
 #define QUEUE_FLAG_QUIESCED    28	/* queue has been quiesced */
 #define QUEUE_FLAG_PREEMPT_ONLY	29	/* only process REQ_PREEMPT requests */
+#define QUEUE_FLAG_DEFER_REG	30	/* defer registering queue to a disk */
 
 #define QUEUE_FLAG_DEFAULT	((1 << QUEUE_FLAG_IO_STAT) |		\
 				 (1 << QUEUE_FLAG_SAME_COMP)	|	\
-- 
2.15.0