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From:   Qian Cai <cai@lca.pw>
To:     akpm@linux-foundation.org
Cc:     penguin-kernel@i-love.sakura.ne.jp, hannes@cmpxchg.org,
        elver@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Qian Cai <cai@lca.pw>,
        syzbot+f36cfe60b1006a94f9dc@syzkaller.appspotmail.com
Subject: [PATCH -next] mm/page_counter: annotate an intentional data race
Date:   Wed, 29 Jan 2020 08:12:42 -0500
Message-Id: <20200129131242.4329-1-cai@lca.pw>
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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@syzkaller.appspotmail.com
Signed-off-by: Qian Cai <cai@lca.pw>
---
 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)