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

[Christoph Hellwig <hch@infradead.org>]

On Mon, Dec 13, 2021 at 12:09:07PM +0800, Laibin Qiu wrote:
> Submit_bio
>                                                          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 judge whether QUEUE_FLAG_REGISTERED is marked to distinguish
> scsi remove scene.
> Fixes: 76a8040817b4b ("blk-wbt: make sure throttle is enabled properly")
> Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>
> ---
>  block/blk-wbt.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/block/blk-wbt.c b/block/blk-wbt.c
> index 3ed71b8da887..537f77bb1365 100644
> --- a/block/blk-wbt.c
> +++ b/block/blk-wbt.c
> @@ -637,6 +637,10 @@ void wbt_enable_default(struct request_queue *q)
>  {
>  	struct rq_qos *rqos = wbt_rq_qos(q);
>  
> +	/* Queue not registered? Maybe shutting down... */
> +	if (!blk_queue_registered(q))
> +		return;

Wouldn't it make more sense to simply not call wbt_enable_default from
elv_unregister_queue?