[dm-devel] [for-4.16 PATCH 1/2] block: cope with gendisk's 'queue' being added later

[Mike Snitzer]

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 was block/genhd.c:add_disk() has required
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:

- Adjust device_add_disk() so that that it can cope with the gendisk _not_
  having its 'queue' established yet.

- Remove del_gendisk()'s WARN_ON() if disk->queue is NULL

- Move "bdi" symlink creation from register_disk() to the end of
  blk_register_queue() -- it is more logical in that the bdi is part of
  the request_queue.

- Move extra request_queue reference count (on behalf of gendisk) from
  device_add_disk() to end of blk_register_queue().

- Make device_add_disk()'s calls to bdi_register_owner() and
  blk_register_queue() conditional on disk->queue not being NULL.

- 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.

These changes have been tested to work without any IO races because the
request_queue and associated bdi don't even exist at the time that the
gendisk's "struct device"s are established by device_add_disk().  I've
been mindful of historic bugs, and haven't experienced them with DM,
e.g.: https://bugzilla.kernel.org/show_bug.cgi?id=16312 (fixed by commit
01ea5063 "block: Fix race during disk initialization")

Signed-off-by: Mike Snitzer <snitzer at redhat.com>
---
 block/blk-sysfs.c | 12 ++++++++++++
 block/genhd.c     | 28 ++++++++--------------------
 2 files changed, 20 insertions(+), 20 deletions(-)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 870484eaed1f..0b0dda8e2420 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -919,8 +919,20 @@ int blk_register_queue(struct gendisk *disk)
 	ret = 0;
 unlock:
 	mutex_unlock(&q->sysfs_lock);
+
+	/*
+	 * Take an extra ref on queue which will be put on disk_release()
+	 * so that it sticks around as long as @disk is there.
+	 */
+	WARN_ON_ONCE(!blk_get_queue(q));
+
+	WARN_ON(sysfs_create_link(&dev->kobj,
+				  &q->backing_dev_info->dev->kobj,
+				  "bdi"));
+
 	return ret;
 }
+EXPORT_SYMBOL_GPL(blk_register_queue);
 
 void blk_unregister_queue(struct gendisk *disk)
 {
diff --git a/block/genhd.c b/block/genhd.c
index 96a66f671720..13aa80319b3b 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -621,11 +621,6 @@ static void register_disk(struct device *parent, struct gendisk *disk)
 	while ((part = disk_part_iter_next(&piter)))
 		kobject_uevent(&part_to_dev(part)->kobj, KOBJ_ADD);
 	disk_part_iter_exit(&piter);
-
-	err = sysfs_create_link(&ddev->kobj,
-				&disk->queue->backing_dev_info->dev->kobj,
-				"bdi");
-	WARN_ON(err);
 }
 
 /**
@@ -671,24 +666,19 @@ void device_add_disk(struct device *parent, struct gendisk *disk)
 		disk->flags |= GENHD_FL_SUPPRESS_PARTITION_INFO;
 		disk->flags |= GENHD_FL_NO_PART_SCAN;
 	} else {
-		int ret;
-
-		/* Register BDI before referencing it from bdev */
 		disk_to_dev(disk)->devt = devt;
-		ret = bdi_register_owner(disk->queue->backing_dev_info,
-						disk_to_dev(disk));
-		WARN_ON(ret);
+		/* Register BDI before referencing it from bdev */
+		if (disk->queue) {
+			retval = bdi_register_owner(disk->queue->backing_dev_info,
+						    disk_to_dev(disk));
+			WARN_ON(retval);
+		}
 		blk_register_region(disk_devt(disk), disk->minors, NULL,
 				    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()
-	 * so that it sticks around as long as @disk is there.
-	 */
-	WARN_ON_ONCE(!blk_get_queue(disk->queue));
+	if (disk->queue)
+		blk_register_queue(disk);
 
 	disk_add_events(disk);
 	blk_integrity_add(disk);
@@ -727,8 +717,6 @@ void del_gendisk(struct gendisk *disk)
 		 */
 		bdi_unregister(disk->queue->backing_dev_info);
 		blk_unregister_queue(disk);
-	} else {
-		WARN_ON(1);
 	}
 
 	if (!(disk->flags & GENHD_FL_HIDDEN))
-- 
2.15.0