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

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Date: Fri, 10 May 2024 09:20:58 -0700
From: Bart Van Assche <bvanassche@....org>
To: paulmck@...nel.org
Cc: Breno Leitao <leitao@...ian.org>, Jens Axboe <axboe@...nel.dk>,
 "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 8:41 AM, Paul E. McKenney wrote:
> 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.

Can using data_race() instead of READ_ONCE() result in incorrect code
generation, e.g. the compiler emitting a read twice and reading two
different values?

Thanks,

Bart.