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

Commit Message

Chengming Zhou April 12, 2023, 4:07 p.m. UTC

From: Chengming Zhou <[e-mail]>

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.

Signed-off-by: Chengming Zhou <[e-mail]>
---
 block/blk-stat.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Tejun Heo April 12, 2023, 5:12 p.m. UTC | #1

On Thu, Apr 13, 2023 at 12:07:53AM +0800, [e-mail] wrote:
> From: Chengming Zhou <[e-mail]>
> 
> 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.
> 
> Signed-off-by: Chengming Zhou <[e-mail]>

Acked-by: Tejun Heo <[e-mail]>

It'd be useful to explicitly illustrate the buggy behavior in the
description (e.g. if you do X, Y and then Z, then X incorrectly loses
accounting). Can you also please add the appropriate stable cc?

Thanks.

Chengming Zhou April 13, 2023, 3:23 a.m. UTC | #2

On 2023/4/13 01:12, Tejun Heo wrote:
> On Thu, Apr 13, 2023 at 12:07:53AM +0800, [e-mail] wrote:
>> From: Chengming Zhou <[e-mail]>
>>
>> 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.
>>
>> Signed-off-by: Chengming Zhou <[e-mail]>
> 
> Acked-by: Tejun Heo <[e-mail]>
> 
> It'd be useful to explicitly illustrate the buggy behavior in the
> description (e.g. if you do X, Y and then Z, then X incorrectly loses

Yes, I will add below buggy behavior in the next version:

This bug can be reproduced as below on kernel without BLK_DEV_THROTTLING
(since it will unconditionally enable accounting, see the second 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.


> accounting). Can you also please add the appropriate stable cc?

Ok, will do.

Thanks.

> 
> Thanks.
>

Patch

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);
 }