lkml.org 
[lkml]   [2024]   [May]   [10]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] block: Annotate a racy read in blk_do_io_stat()
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

\
 
 \ /
  Last update: 2024-05-27 18:24    [W:0.069 / U:0.016 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site