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.
