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

[Laibin Qiu]

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 judge whether QUEUE_FLAG_REGISTERED is marked to distinguish
scsi remove scene.
Fixes: 76a8040817b4b ("blk-wbt: make sure throttle is enabled properly")
Signed-off-by: Laibin Qiu <qiulaibin@xxxxxxxxxx>
---
 block/blk-wbt.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index 3ed71b8da887..537f77bb1365 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c
@@ -637,6 +637,10 @@ void wbt_enable_default(struct request_queue *q)
 {
 	struct rq_qos *rqos = wbt_rq_qos(q);
 
+	/* Queue not registered? Maybe shutting down... */
+	if (!blk_queue_registered(q))
+		return;
+
 	/* Throttling already enabled? */
 	if (rqos) {
 		if (RQWB(rqos)->enable_state == WBT_STATE_OFF_DEFAULT)
@@ -644,10 +648,6 @@ void wbt_enable_default(struct request_queue *q)
 		return;
 	}
 
-	/* Queue not registered? Maybe shutting down... */
-	if (!blk_queue_registered(q))
-		return;
-
 	if (queue_is_mq(q) && IS_ENABLED(CONFIG_BLK_WBT_MQ))
 		wbt_init(q);
 }
-- 
2.22.0