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

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Message-ID: <Zj411QK+K+CmXaVi@gmail.com>
Date: Fri, 10 May 2024 07:57:25 -0700
From: Breno Leitao <leitao@...ian.org>
To: Bart Van Assche <bvanassche@....org>
Cc: Jens Axboe <axboe@...nel.dk>, 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 Fri, May 10, 2024 at 07:28:41AM -0700, Bart Van Assche wrote:
> 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()?

data_race() doesn't not emit any code, but, keep KCSAN silent.
READ_ONCE()/WRITE_ONCE() emits code.

So, if you do not want to change the current behaviour, but, keep KCSAN
away, data_race() is preferred.