2020-01-29 13:14:31

by Qian Cai

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

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: [email protected]
Signed-off-by: Qian Cai <[email protected]>
---
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)

2020-01-29 13:32:07

by Michal Hocko

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

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: [email protected]
> Signed-off-by: Qian Cai <[email protected]>

Acked-by: Michal Hocko <[email protected]>

> ---
> 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

2020-02-11 12:44:05

by Qian Cai

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

> On Jan 29, 2020, at 8:29 AM, Michal Hocko <[email protected]> wrote:
>
> Acked-by: Michal Hocko <[email protected]>

Andrew, you might forget about picking this up too?