linux-kernel - Re: [PATCH v3 -next] block/wbt: fix negative inflight counter when remove scsi device

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Date:   Wed, 15 Dec 2021 10:42:37 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Laibin Qiu <qiulaibin@...wei.com>
Cc:     hch@...radead.org, axboe@...nel.dk, yi.zhang@...wei.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 -next] block/wbt: fix negative inflight counter when
 remove scsi device

On Tue, Dec 14, 2021 at 09:31:03PM +0800, Laibin Qiu wrote:
> 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 move wbt_enable_default() from elv_unregister to
> bfq_exit_queue(). Only re-enable wbt when bfq exit.
> Fixes: 76a8040817b4b ("blk-wbt: make sure throttle is enabled properly")
> Signed-off-by: Laibin Qiu <qiulaibin@...wei.com>
> ---
>  block/bfq-iosched.c | 4 ++++
>  block/elevator.c    | 2 --
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 0c612a911696..8b7524450835 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -6996,6 +6996,7 @@ static void bfq_exit_queue(struct elevator_queue *e)
>  {
>  	struct bfq_data *bfqd = e->elevator_data;
>  	struct bfq_queue *bfqq, *n;
> +	struct request_queue *q = bfqd->queue;
>  
>  	hrtimer_cancel(&bfqd->idle_slice_timer);
>  
> @@ -7019,6 +7020,9 @@ static void bfq_exit_queue(struct elevator_queue *e)
>  #endif
>  
>  	kfree(bfqd);
> +
> +	/* Re-enable throttling in case elevator disabled it */

Of course, bfq has disabled it, so the above comment is useless,
otherwise looks fine:

Reviewed-by: Ming Lei <ming.lei@...dat.com>


Thanks,
Ming