Re: [PATCH v2] mm/percpu: prevent concurrency problem for pcpu_nr_populated read with spin lock

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 11 Jul 2025 17:56:34 +0200 Vlastimil Babka <vbabka@suse.cz> wrote:

> On 7/3/25 08:56, Jeongjun Park wrote:
> > pcpu_nr_pages() reads pcpu_nr_populated without any protection, which
> > causes a data race between read/write.
> > 
> > However, since this is an intended race, we should add a data_race
> > annotation instead of add a spin lock.
> > 
> > Reported-by: syzbot+e5bd32b79413e86f389e@syzkaller.appspotmail.com
> > Fixes: 7e8a6304d541 ("/proc/meminfo: add percpu populated pages count")
> > Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
> > Signed-off-by: Jeongjun Park <aha310510@gmail.com>
> > ---
> > v2: Change it as suggested by Shakeel Butt
> > - Link to v1: https://lore.kernel.org/all/20250702082749.141616-1-aha310510@gmail.com/
> > ---
> >  mm/percpu.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index b35494c8ede2..782cc148b39c 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -3355,7 +3355,7 @@ void __init setup_per_cpu_areas(void)
> >   */
> >  unsigned long pcpu_nr_pages(void)
> >  {
> > -	return pcpu_nr_populated * pcpu_nr_units;
> > +	return data_race(READ_ONCE(pcpu_nr_populated) * pcpu_nr_units);
> 
> Nit: pcpu_nr_units should be excluded from the scope of data_race() as no
> race can happen there.

This?

--- a/mm/percpu.c~mm-percpu-prevent-concurrency-problem-for-pcpu_nr_populated-read-with-spin-lock-fix
+++ a/mm/percpu.c
@@ -3355,7 +3355,7 @@ void __init setup_per_cpu_areas(void)
  */
 unsigned long pcpu_nr_pages(void)
 {
-	return data_race(READ_ONCE(pcpu_nr_populated) * pcpu_nr_units);
+	return data_race(READ_ONCE(pcpu_nr_populated)) * pcpu_nr_units;
 }
 
 /*
_