[PATCH v2 -next] block/wbt: fix negative inflight counter when remove scsi device

Laibin Qiu
2021-12-14T04:27:48Z

Now that we disable wbt by set WBT_STATE_OFF_DEFAULT in
wbt_disable_default() when switch elevator to bfq. And when
we remove scsi device, wbt will be enabled by wbt_enable_default.
If it become false positive between wbt_wait() and wbt_track()
when submit write request.

The following is the scenario that triggered the problem.

T1 T2 T3
elevator_switch_mq
bfq_init_queue
wbt_disable_default <= Set
rwb->enable_state (OFF)
Submit_bio
blk_mq_make_request
rq_qos_throttle
<= rwb->enable_state (OFF)
scsi_remove_device
sd_remove
del_gendisk
blk_unregister_queue
elv_unregister_queue
wbt_enable_default
<= Set rwb->enable_state (ON)
q_qos_track
<= rwb->enable_state (ON)
^^^^^^ this request will mark WBT_TRACKED without inflight add and will
lead to drop rqw->inflight to -1 in wbt_done() which will trigger IO hung.

Fix this by move wbt_enable_default() from elv_unregister to
elevator_switch_mq. Only re-enable wbt when scheduler switch.
Fixes: 76a8040817b4b ("blk-wbt: make sure throttle is enabled properly")
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>
---
block/elevator.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/block/elevator.c b/block/elevator.c
index ec98aed39c4f..de3cf1fa52fa 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -525,8 +525,6 @@ 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);
}
}

@@ -593,8 +591,11 @@ 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);
blk_mq_sched_free_rqs(q);
--
2.22.0