block: add new helper for disabling elevator switch when deleting disk

Add new helper disable_elv_switch() and new flag QUEUE_FLAG_NO_ELV_SWITCH
for disabling elevator switch before deleting disk:

- originally flag QUEUE_FLAG_REGISTERED is added for preventing elevator
switch during removing disk, but this flag has been used widely for
other purposes, so add one new flag for disabling elevator switch only

- for avoiding deadlock risk, we have to move elevator queue
register/unregister out of elevator lock and queue freeze, which will be
done in next patch. However, this way adds small race window between elevator
switch and deleting ->queue_kobj, in which elevator queue register/unregister
could be run concurrently. The added helper will be used for avoiding the race
in the following patch.

- drain in-progress elevator switch before deleting disk

Suggested-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Link: https://lore.kernel.org/r/20250505141805.2751237-21-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 13 insertions, 0 deletions

diff --git a/block/genhd.c b/block/genhd.c
index f192fe4808b9..a8cb5607b6e3 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -764,6 +764,16 @@ static void __del_gendisk(struct gendisk *disk)
 		blk_unfreeze_release_lock(q);
 }
 
+static void disable_elv_switch(struct request_queue *q)
+{
+	struct blk_mq_tag_set *set = q->tag_set;
+	WARN_ON_ONCE(!queue_is_mq(q));
+
+	down_write(&set->update_nr_hwq_lock);
+	blk_queue_flag_set(QUEUE_FLAG_NO_ELV_SWITCH, q);
+	up_write(&set->update_nr_hwq_lock);
+}
+
 /**
  * del_gendisk - remove the gendisk
  * @disk: the struct gendisk to remove
@@ -792,6 +802,9 @@ void del_gendisk(struct gendisk *disk)
 		__del_gendisk(disk);
 	} else {
 		set = disk->queue->tag_set;
+
+		disable_elv_switch(disk->queue);
+
 		memflags = memalloc_noio_save();
 		down_read(&set->update_nr_hwq_lock);
 		__del_gendisk(disk);