Re: [PATCH -next] mm/page_counter: annotate an intentional data race

[Michal Hocko]

On Wed 29-01-20 08:12:42, Qian Cai wrote:
> The commit 3e32cb2e0a12 ("mm: memcontrol: lockless page counters") could
> had memcg->memsw->failcnt been accessed concurrently as reported by
> KCSAN,
> 
> BUG: KCSAN: data-race in page_counter_try_charge / page_counter_try_charge
> 
> write to 0xffff88809bbf2158 of 8 bytes by task 11782 on cpu 0:
>  page_counter_try_charge+0x100/0x170 mm/page_counter.c:129
>  try_charge+0x185/0xbf0 mm/memcontrol.c:2405
>  __memcg_kmem_charge_memcg+0x4a/0xe0 mm/memcontrol.c:2837
>  __memcg_kmem_charge+0xcf/0x1b0 mm/memcontrol.c:2877
>  __alloc_pages_nodemask+0x26c/0x310 mm/page_alloc.c:4780
> 
> read to 0xffff88809bbf2158 of 8 bytes by task 11814 on cpu 1:
>  page_counter_try_charge+0xef/0x170 mm/page_counter.c:129
>  try_charge+0x185/0xbf0 mm/memcontrol.c:2405
>  __memcg_kmem_charge_memcg+0x4a/0xe0 mm/memcontrol.c:2837
>  __memcg_kmem_charge+0xcf/0x1b0 mm/memcontrol.c:2877
>  __alloc_pages_nodemask+0x26c/0x310 mm/page_alloc.c:4780
> 
> Since the "failcnt" counter is tolerant of some degree of inaccuracy and
> is only used to report stats, a data race will not be harmful, thus mark
> it as an intentional data races with the data_race() macro.
> 
> Reported-by: syzbot+f36cfe60b1006a94f9dc@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Qian Cai <cai@xxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/page_counter.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_counter.c b/mm/page_counter.c
> index a17841150906..7c82072cda25 100644
> --- a/mm/page_counter.c
> +++ b/mm/page_counter.c
> @@ -126,7 +126,7 @@ bool page_counter_try_charge(struct page_counter *counter,
>  			 * This is racy, but we can live with some
>  			 * inaccuracy in the failcnt.
>  			 */
> -			c->failcnt++;
> +			data_race(c->failcnt++);
>  			*fail = c;
>  			goto failed;
>  		}
> -- 
> 2.21.0 (Apple Git-122.2)

-- 
Michal Hocko
SUSE Labs