block: fix race between wbt_enable_default and IO submission

When wbt_enable_default() is moved out of queue freezing in elevator_change(),
it can cause the wbt inflight counter to become negative (-1), leading to hung
tasks in the writeback path. Tasks get stuck in wbt_wait() because the counter
is in an inconsistent state.

The issue occurs because wbt_enable_default() could race with IO submission,
allowing the counter to be decremented before proper initialization. This manifests
as:

rq_wait[0]:
inflight: -1
has_waiters: True

rwb_enabled() checks the state, which can be updated exactly between wbt_wait()
(rq_qos_throttle()) and wbt_track()(rq_qos_track()), then the inflight counter
will become negative.

And results in hung task warnings like:
task:kworker/u24:39 state:D stack:0 pid:14767
Call Trace:
rq_qos_wait+0xb4/0x150
wbt_wait+0xa9/0x100
__rq_qos_throttle+0x24/0x40
blk_mq_submit_bio+0x672/0x7b0
...

Fix this by:

1. Splitting wbt_enable_default() into:
- __wbt_enable_default(): Returns true if wbt_init() should be called
- wbt_enable_default(): Wrapper for existing callers (no init)
- wbt_init_enable_default(): New function that checks and inits WBT

2. Using wbt_init_enable_default() in blk_register_queue() to ensure
proper initialization during queue registration

3. Move wbt_init() out of wbt_enable_default() which is only for enabling
disabled wbt from bfq and iocost, and wbt_init() isn't needed. Then the
original lock warning can be avoided.

4. Removing the ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT flag and its handling
code since it's no longer needed

This ensures WBT is properly initialized before any IO can be submitted,
preventing the counter from going negative.

Cc: Nilay Shroff <[e-mail]>
Cc: Yu Kuai <[e-mail]>
Cc: Guangwu Zhang <[e-mail]>
Fixes: 78c271344b6f ("block: move wbt_enable_default() out of queue freezing from sched ->exit()")
Signed-off-by: Ming Lei <[e-mail]>
Reviewed-by: Nilay Shroff <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/bfq-iosched.c

···
 
 	blk_stat_disable_accounting(bfqd->queue);
 	blk_queue_flag_clear(QUEUE_FLAG_DISABLE_WBT_DEF, bfqd->queue);
-	set_bit(ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT, &e->flags);
+	wbt_enable_default(bfqd->queue->disk);
 
 	kfree(bfqd);
 }

block/blk-sysfs.c

···
 		elevator_set_default(q);
 
 	blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);
-	wbt_enable_default(disk);
+	wbt_init_enable_default(disk);
 
 	/* Now everything is ready and send out KOBJ_ADD uevent */
 	kobject_uevent(&disk->queue_kobj, KOBJ_ADD);

block/blk-wbt.c

···
 /*
  * Enable wbt if defaults are configured that way
  */
-void wbt_enable_default(struct gendisk *disk)
+static bool __wbt_enable_default(struct gendisk *disk)
 {
 	struct request_queue *q = disk->queue;
 	struct rq_qos *rqos;
···
 		if (enable && RQWB(rqos)->enable_state == WBT_STATE_OFF_DEFAULT)
 			RQWB(rqos)->enable_state = WBT_STATE_ON_DEFAULT;
 		mutex_unlock(&disk->rqos_state_mutex);
-		return;
+		return false;
 	}
 	mutex_unlock(&disk->rqos_state_mutex);
 
 	/* Queue not registered? Maybe shutting down... */
 	if (!blk_queue_registered(q))
-		return;
+		return false;
 
 	if (queue_is_mq(q) && enable)
-		wbt_init(disk);
+		return true;
+	return false;
+}
+
+void wbt_enable_default(struct gendisk *disk)
+{
+	__wbt_enable_default(disk);
 }
 EXPORT_SYMBOL_GPL(wbt_enable_default);
+
+void wbt_init_enable_default(struct gendisk *disk)
+{
+	if (__wbt_enable_default(disk))
+		WARN_ON_ONCE(wbt_init(disk));
+}
 
 u64 wbt_default_latency_nsec(struct request_queue *q)
 {

block/blk-wbt.h

···
 #ifdef CONFIG_BLK_WBT
 
 int wbt_init(struct gendisk *disk);
+void wbt_init_enable_default(struct gendisk *disk);
 void wbt_disable_default(struct gendisk *disk);
 void wbt_enable_default(struct gendisk *disk);
 
···
 u64 wbt_default_latency_nsec(struct request_queue *);
 
 #else
+
+static inline void wbt_init_enable_default(struct gendisk *disk)
+{
+}
 
 static inline void wbt_disable_default(struct gendisk *disk)
 {

block/elevator.c

···
 			.et = ctx->old->et,
 			.data = ctx->old->elevator_data
 		};
-		bool enable_wbt = test_bit(ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT,
-				&ctx->old->flags);
 
 		elv_unregister_queue(q, ctx->old);
 		blk_mq_free_sched_res(&res, ctx->old->type, q->tag_set);
 		kobject_put(&ctx->old->kobj);
-		if (enable_wbt)
-			wbt_enable_default(q->disk);
 	}
 	if (ctx->new) {
 		ret = elv_register_queue(q, ctx->new, !ctx->no_uevent);

block/elevator.h

···
 
 #define ELEVATOR_FLAG_REGISTERED	0
 #define ELEVATOR_FLAG_DYING		1
-#define ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT	2
 
 /*
  * block elevator interface