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Date: Fri, 10 May 2024 08:41:07 -0700
From: "Paul E. McKenney" <paulmck@kernel.org>
To: Bart Van Assche <bvanassche@acm.org>
Cc: Breno Leitao <leitao@debian.org>, Jens Axboe <axboe@kernel.dk>,
	"open list:BLOCK LAYER" <linux-block@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] block: Annotate a racy read in blk_do_io_stat()
Message-ID: <de92101c-f9c4-4af4-95f4-19a6f59b636f@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <20240510141921.883231-1-leitao@debian.org>
 <ef8c5f6d-17e3-4504-8560-b970912b9eae@acm.org>
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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()?

We use this pattern quite a bit in RCU.  For example, suppose that we
have a variable that is accessed only under a given lock, except that it
is also locklessly accessed for diagnostics or statistics.  Then having
unmarked (normal C language) accesses under the lock and data_race()
for that statistics enables KCSAN to flag other (buggy) lockless accesses.

							Thanx, Paul