[RFC 2/6] block: add flag for add_disk() completion notation

[Luis Chamberlain <mcgrof@kernel.org>]

Often drivers may have complex setups where it is not
clear if their disk completed their respective *add_disk*()
call. They either have to invent a setting or, they
incorrectly use GENHD_FL_UP. Using GENHD_FL_UP however is
used internally so we know when we can add / remove
partitions safely. We can easily fail along the way
prior to add_disk() completing and still have
GENHD_FL_UP set, so it would not be correct in that case
to call del_gendisk() on the disk.

Provide a new flag then which allows us to check if
*add_disk*() completed, and conversely just make
del_gendisk() check for this for drivers so that
they can safely call del_gendisk() and we'll figure
it out if it is safe for you to call this.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
---
 block/genhd.c         |  8 ++++++++
 include/linux/genhd.h | 11 ++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/block/genhd.c b/block/genhd.c
index c6c9c196ff27..72703d243b44 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -630,6 +630,8 @@ static int __device_add_disk(struct device *parent, struct gendisk *disk,
 	if (ret)
 		goto exit_del_events;
 
+	disk->flags |= GENHD_FL_DISK_ADDED;
+
 	return 0;
 exit_del_events:
 	disk_del_events(disk);
@@ -677,6 +679,9 @@ EXPORT_SYMBOL(device_add_disk_no_queue_reg);
  * with put_disk(), which should be called after del_gendisk(), if
  * __device_add_disk() was used.
  *
+ * Drivers can safely call this even if they are not sure if the respective
+ * __device_add_disk() call succeeded.
+ *
  * Drivers exist which depend on the release of the gendisk to be synchronous,
  * it should not be deferred.
  *
@@ -686,6 +691,9 @@ void del_gendisk(struct gendisk *disk)
 {
 	might_sleep();
 
+	if (!blk_disk_registered(disk))
+		return;
+
 	if (WARN_ON_ONCE(!disk->queue))
 		return;
 
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index dc07a957c9e1..73024416d2d5 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -56,6 +56,10 @@ struct partition_meta_info {
  * Must not be set for devices which are removed entirely when the
  * media is removed.
  *
+ * ``GENHD_FL_DISK_ADDED`` (0x0002): used to clarify that the
+ * respective add_disk*() call completed successfully, so that
+ * we know we can safely process del_gendisk() on the disk.
+ *
  * ``GENHD_FL_CD`` (0x0008): the block device is a CD-ROM-style
  * device.
  * Affects responses to the ``CDROM_GET_CAPABILITY`` ioctl.
@@ -94,7 +98,7 @@ struct partition_meta_info {
  * Used for multipath devices.
  */
 #define GENHD_FL_REMOVABLE			0x0001
-/* 2 is unused (used to be GENHD_FL_DRIVERFS) */
+#define GENHD_FL_DISK_ADDED			0x0002
 /* 4 is unused (used to be GENHD_FL_MEDIA_CHANGE_NOTIFY) */
 #define GENHD_FL_CD				0x0008
 #define GENHD_FL_UP				0x0010
@@ -189,6 +193,11 @@ struct gendisk {
 #define disk_to_cdi(disk)	NULL
 #endif
 
+static inline bool blk_disk_registered(struct gendisk *disk)
+{
+	return disk && (disk->flags & GENHD_FL_DISK_ADDED);
+}
+
 static inline int disk_max_parts(struct gendisk *disk)
 {
 	if (disk->flags & GENHD_FL_EXT_DEVT)
-- 
2.27.0