[PATCH -next v2] locking/osq_lock: annotate a data race in osq_lock

[Qian Cai]

prev->next could be accessed concurrently as noticed by KCSAN,

 write (marked) to 0xffff9d3370dbbe40 of 8 bytes by task 3294 on cpu 107:
  osq_lock+0x25f/0x350
  osq_wait_next at kernel/locking/osq_lock.c:79
  (inlined by) osq_lock at kernel/locking/osq_lock.c:185
  rwsem_optimistic_spin
  <snip>

 read to 0xffff9d3370dbbe40 of 8 bytes by task 3398 on cpu 100:
  osq_lock+0x196/0x350
  osq_lock at kernel/locking/osq_lock.c:157
  rwsem_optimistic_spin
  <snip>

Since the write only stores NULL to prev->next and the read tests if
prev->next equals to this_cpu_ptr(&osq_node). Even if the value is
shattered, the code is still working correctly. Thus, mark it as an
intentional data race using the data_race() macro.

Signed-off-by: Qian Cai <cai@xxxxxx>
---

v2: insert some code comments.

 kernel/locking/osq_lock.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 1f7734949ac8..f733bcd99e8a 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -154,7 +154,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 */
 
 	for (;;) {
-		if (prev->next == node &&
+		/*
+		 * cpu_relax() below implies a compiler barrier which would
+		 * prevent this comparison being optimized away.
+		 */
+		if (data_race(prev->next == node) &&
 		    cmpxchg(&prev->next, node, NULL) == node)
 			break;
 
-- 
1.8.3.1