linux-kernel - Re: [PATCH v2 1/2] blk-throtl: move WARN_ON_ONCE() from throtl_rb

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Date:   Mon, 29 Nov 2021 22:43:13 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     hch@...radead.org, tj@...nel.org, axboe@...nel.dk,
        cgroups@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH v2 1/2] blk-throtl: move WARN_ON_ONCE() from
 throtl_rb_first() to it's caller

On Tue, Nov 30, 2021 at 09:17:29AM +0800, Yu Kuai wrote:
> Prepare to reintroduce tg_drain_bios(), which will iterate until
> throtl_rb_first() return NULL.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>  block/blk-throttle.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> index 39bb6e68a9a2..f7244190cb2f 100644
> --- a/block/blk-throttle.c
> +++ b/block/blk-throttle.c
> @@ -502,7 +502,6 @@ throtl_rb_first(struct throtl_service_queue *parent_sq)
>  	struct rb_node *n;
>  
>  	n = rb_first_cached(&parent_sq->pending_tree);
> -	WARN_ON_ONCE(!n);
>  	if (!n)
>  		return NULL;
>  	return rb_entry_tg(n);
> @@ -521,8 +520,10 @@ static void update_min_dispatch_time(struct throtl_service_queue *parent_sq)
>  	struct throtl_grp *tg;
>  
>  	tg = throtl_rb_first(parent_sq);
> -	if (!tg)
> +	if (!tg) {
> +		WARN_ON_ONCE(1);
>  		return;

	if (WARN_ON_ONCE(!tg))
		return;

>  		tg = throtl_rb_first(parent_sq);
> -		if (!tg)
> +		if (!tg) {
> +			WARN_ON_ONCE(1);
>  			break;
> +		}

Same here.