[PATCH] locking/osq_lock: fix a data race in osq_wait_next

[Qian Cai]

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@xxxxxx>
---
 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)