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    SubjectRe: [PATCH] block: Annotate a racy read in blk_do_io_stat()
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    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.


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