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From: Qian Cai <cai@lca.pw>
To: mingo@redhat.com
Cc: peterz@infradead.org, will@kernel.org, elver@google.com,
	linux-kernel@vger.kernel.org, Qian Cai <cai@lca.pw>
Subject: [PATCH] locking/osq_lock: fix a data race in osq_wait_next
Date: Wed, 22 Jan 2020 11:38:57 -0500
Message-ID: <20200122163857.4605-1-cai@lca.pw> (raw)

KCSAN complains,

 write (marked) to 0xffff941ca3b3be00 of 8 bytes by task 670 on cpu 6:
  osq_lock+0x24c/0x340
  __mutex_lock+0x277/0xd20
  mutex_lock_nested+0x31/0x40
  memcg_create_kmem_cache+0x2e/0x190
  memcg_kmem_cache_create_func+0x40/0x80
  process_one_work+0x54c/0xbe0
  worker_thread+0x80/0x650
  kthread+0x1e0/0x200
  ret_from_fork+0x27/0x50

 read to 0xffff941ca3b3be00 of 8 bytes by task 703 on cpu 44:
  osq_lock+0x18e/0x340
  __mutex_lock+0x277/0xd20
  mutex_lock_nested+0x31/0x40
  memcg_create_kmem_cache+0x2e/0x190
  memcg_kmem_cache_create_func+0x40/0x80
  process_one_work+0x54c/0xbe0
  worker_thread+0x80/0x650
  kthread+0x1e0/0x200
  ret_from_fork+0x27/0x50

which points to those lines in osq_wait_next(),

  next = xchg(&node->next, NULL);
  if (next)
	break;

Since only the read is outside of critical sections, fixed it by adding
a READ_ONCE().

Signed-off-by: Qian Cai <cai@lca.pw>
---
 kernel/locking/osq_lock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 6ef600aa0f47..8f565165019a 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -77,7 +77,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
 		 */
 		if (node->next) {
 			next = xchg(&node->next, NULL);
-			if (next)
+			if (READ_ONCE(next))
 				break;
 		}
 
-- 
2.21.0 (Apple Git-122.2)


             reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-22 16:38 Qian Cai [this message]
2020-01-22 16:59 ` Will Deacon
2020-01-22 17:08   ` Qian Cai
2020-01-22 22:38     ` Marco Elver
2020-01-22 23:54       ` Qian Cai
2020-01-23  9:39         ` Peter Zijlstra
2020-01-28  3:11           ` Qian Cai
2020-01-28 11:46             ` Marco Elver
2020-01-28 12:53               ` Qian Cai
2020-01-28 16:52               ` Peter Zijlstra
2020-01-28 16:56               ` Peter Zijlstra
2020-01-29  0:22                 ` Paul E. McKenney
2020-01-29 15:29                   ` Marco Elver
2020-01-29 18:40                     ` Peter Zijlstra
2020-01-30 13:39                       ` Marco Elver
2020-01-30 13:48                         ` Peter Zijlstra
2020-01-31  3:32                           ` Qian Cai
2020-01-29 18:49                   ` Peter Zijlstra
2020-01-29 19:26                     ` Paul E. McKenney
2020-01-23  9:36       ` Peter Zijlstra
2020-01-28  3:12         ` Qian Cai
2020-01-28  8:18           ` Marco Elver
2020-01-28 10:10             ` Qian Cai
2020-01-28 10:29               ` Marco Elver
2020-01-22 17:09 ` Peter Zijlstra

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