linux-kernel - Re: [PATCH] block: Annotate a racy read in blk_do_io

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Date: Fri, 10 May 2024 07:28:41 -0700
From: Bart Van Assche <bvanassche@....org>
To: Breno Leitao <leitao@...ian.org>, Jens Axboe <axboe@...nel.dk>
Cc: paulmck@...nel.org, "open list:BLOCK LAYER"
 <linux-block@...r.kernel.org>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: Annotate a racy read in blk_do_io_stat()

On 5/10/24 07:19, Breno Leitao wrote:
> diff --git a/block/blk.h b/block/blk.h
> index d9f584984bc4..57a1d73a0718 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -353,7 +353,8 @@ int blk_dev_init(void);
>    */
>   static inline bool blk_do_io_stat(struct request *rq)
>   {
> -	return (rq->rq_flags & RQF_IO_STAT) && !blk_rq_is_passthrough(rq);
> +	/* Disk stats reading isn’t critical, let it race */
> +	return (data_race(rq->rq_flags) & RQF_IO_STAT) && !blk_rq_is_passthrough(rq);
>   }
>   
>   void update_io_ticks(struct block_device *part, unsigned long now, bool end);

Why to annotate this race with data_race() instead of READ_ONCE()? Are
there any cases in which it is better to use data_race() than
READ_ONCE()?

Thanks,

Bart.