linux-kernel - Re: [PATCH -next] blk-iocost: fix sleeping in atomic warning for wbt_enable

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Date:   Wed, 8 Feb 2023 09:06:01 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Yu Kuai <yukuai1@...weicloud.com>
Cc:     josef@...icpanda.com, axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yukuai3@...wei.com, yi.zhang@...wei.com, yangerkun@...wei.com
Subject: Re: [PATCH -next] blk-iocost: fix sleeping in atomic warning for
 wbt_enable_default()

On Wed, Feb 08, 2023 at 11:48:03AM +0800, Yu Kuai wrote:
> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index 7a2dc9dc8e3b..03bfe1dda07c 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -3279,11 +3279,9 @@ static ssize_t ioc_qos_write(struct kernfs_open_file *of, char *input,
>  		blk_stat_enable_accounting(disk->queue);
>  		blk_queue_flag_set(QUEUE_FLAG_RQ_ALLOC_TIME, disk->queue);
>  		ioc->enabled = true;
> -		wbt_disable_default(disk);
>  	} else {
>  		blk_queue_flag_clear(QUEUE_FLAG_RQ_ALLOC_TIME, disk->queue);
>  		ioc->enabled = false;
> -		wbt_enable_default(disk);
>  	}
>  
>  	if (user) {
> @@ -3296,6 +3294,10 @@ static ssize_t ioc_qos_write(struct kernfs_open_file *of, char *input,
>  	ioc_refresh_params(ioc, true);
>  	spin_unlock_irq(&ioc->lock);
>  
> +	if (enable)
> +		wbt_disable_default(disk);
> +	else
> +		wbt_enable_default(disk);

Wouldn't this allow two competiting config attempts to race each other and
leave wbt in an unexpected state?

	task1				task2

	ioc_qos_write()			ioc_qos_write()
	lock()
	enable
	unlock()
					lock()
					disable
					unlock()
					wbt_enable_default()
	wbt_disable_default()

Thanks.

-- 
tejun