block: restore default wbt enablement

The commit 245618f ("block: protect wbt_lat_usec using
q->elevator_lock") protected wbt_enable_default() with
q->elevator_lock; however, it also placed wbt_enable_default()
before blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);, resulting
in wbt failing to be enabled.

Moreover, the protection of wbt_enable_default() by q->elevator_lock
was removed in commit 78c2713 ("block: move wbt_enable_default()
out of queue freezing from sched ->exit()"), so we can directly fix
this issue by placing wbt_enable_default() after
blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);.

Additionally, this issue also causes the inability to read the
wbt_lat_usec file, and the scenario is as follows:

root@q:/sys/block/sda/queue# cat wbt_lat_usec
cat: wbt_lat_usec: Invalid argument

root@q:/data00/sjc/linux# ls /sys/kernel/debug/block/sda/rqos
cannot access '/sys/kernel/debug/block/sda/rqos': No such file or directory

root@q:/data00/sjc/linux# find /sys -name wbt
/sys/kernel/debug/tracing/events/wbt

After testing with this patch, wbt can be enabled normally.

Signed-off-by: Julian Sun <[email protected]>
Cc: [email protected]
Fixes: 245618f ("block: protect wbt_lat_usec using q->elevator_lock")
Reviewed-by: Nilay Shroff <[email protected]>
Reviewed-by: Yu Kuai <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: jc2870

block/blk-sysfs.c

@@ -902,9 +902,9 @@ int blk_register_queue(struct gendisk *disk)
 
 	if (queue_is_mq(q))
 		elevator_set_default(q);
-	wbt_enable_default(disk);
 
 	blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);
+	wbt_enable_default(disk);
 
 	/* Now everything is ready and send out KOBJ_ADD uevent */
 	kobject_uevent(&disk->queue_kobj, KOBJ_ADD);