[PATCH v2 1/2] blk-stat: fix QUEUE_FLAG_STATS clear

[chengming.zhou@linux.dev]

From: Chengming Zhou <zhouchengming@bytedance.com>

We need to set QUEUE_FLAG_STATS for two cases:
1. blk_stat_enable_accounting()
2. blk_stat_add_callback()

So we should clear it only when ((q->stats->accounting == 0) &&
list_empty(&q->stats->callbacks)).

blk_stat_disable_accounting() only check if q->stats->accounting
is 0 before clear the flag, this patch fix it.

Also add list_empty(&q->stats->callbacks)) check when enable, or
the flag is already set.

The bug can be reproduced on kernel without BLK_DEV_THROTTLING
(since it unconditionally enable accounting, see the next patch).

  # cat /sys/block/sr0/queue/scheduler
  none mq-deadline [bfq]

  # cat /sys/kernel/debug/block/sr0/state
  SAME_COMP|IO_STAT|INIT_DONE|STATS|REGISTERED|NOWAIT|30

  # echo none > /sys/block/sr0/queue/scheduler

  # cat /sys/kernel/debug/block/sr0/state
  SAME_COMP|IO_STAT|INIT_DONE|REGISTERED|NOWAIT

  # cat /sys/block/sr0/queue/wbt_lat_usec
  75000

We can see that after changing elevator from "bfq" to "none",
"STATS" flag is lost even though WBT callback still need it.

Fixes: 68497092bde9 ("block: make queue stat accounting a reference")
Cc: <stable@vger.kernel.org> # v5.17+
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Acked-by: Tejun Heo <tj@kernel.org>
---
 block/blk-stat.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/blk-stat.c b/block/blk-stat.c
index 74a1a8c32d86..bc7e0ed81642 100644
--- a/block/blk-stat.c
+++ b/block/blk-stat.c
@@ -190,7 +190,7 @@ void blk_stat_disable_accounting(struct request_queue *q)
 	unsigned long flags;
 
 	spin_lock_irqsave(&q->stats->lock, flags);
-	if (!--q->stats->accounting)
+	if (!--q->stats->accounting && list_empty(&q->stats->callbacks))
 		blk_queue_flag_clear(QUEUE_FLAG_STATS, q);
 	spin_unlock_irqrestore(&q->stats->lock, flags);
 }
@@ -201,7 +201,7 @@ void blk_stat_enable_accounting(struct request_queue *q)
 	unsigned long flags;
 
 	spin_lock_irqsave(&q->stats->lock, flags);
-	if (!q->stats->accounting++)
+	if (!q->stats->accounting++ && list_empty(&q->stats->callbacks))
 		blk_queue_flag_set(QUEUE_FLAG_STATS, q);
 	spin_unlock_irqrestore(&q->stats->lock, flags);
 }
-- 
2.39.2