Re: [bug report][bisected] WARNING: CPU: 109 PID: 739473 at block/blk-stat.c:218 blk_free_queue_stats+0x3c/0x80

[Christoph Hellwig <hch@lst.de>]

Hi Yi,

can you try the patch below?  This changes the teardown code to not
re-enable writeback tracking when we're shutting the queue down, which
is what I suspect is on the ->callbacks list.

diff --git a/block/elevator.c b/block/elevator.c
index ff45d8388f487..bb5c6ee4546cd 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -523,8 +523,6 @@ void elv_unregister_queue(struct request_queue *q)
 		kobject_del(&e->kobj);
 
 		e->registered = 0;
-		/* Re-enable throttling in case elevator disabled it */
-		wbt_enable_default(q);
 	}
 }
 
@@ -591,8 +589,11 @@ int elevator_switch_mq(struct request_queue *q,
 	lockdep_assert_held(&q->sysfs_lock);
 
 	if (q->elevator) {
-		if (q->elevator->registered)
+		if (q->elevator->registered) {
 			elv_unregister_queue(q);
+			/* Re-enable throttling in case elevator disabled it */
+			wbt_enable_default(q);
+		}
 
 		ioc_clear_queue(q);
 		elevator_exit(q, q->elevator);