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); }