Messages in this thread | | | Date | Fri, 10 May 2024 09:20:58 -0700 | Subject | Re: [PATCH] block: Annotate a racy read in blk_do_io_stat() | From | Bart Van Assche <> |
| |
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.
| |