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