md: use RCU lock to protect traversal in md_spares_need_change()

Since md_start_sync() will be called without the protect of mddev_lock,
and it can run concurrently with array reconfiguration, traversal of rdev
in it should be protected by RCU lock.
Commit bc08041b ("md: suspend array in md_start_sync() if array need
reconfiguration") added md_spares_need_change() to md_start_sync(),
casusing use of rdev without any protection.
Fix this by adding RCU lock in md_spares_need_change().

Fixes: bc08041b ("md: suspend array in md_start_sync() if array need reconfiguration")
Cc: stable@vger.kernel.org # 6.7+
Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240104133629.1277517-1-lilingfeng@huaweicloud.com

drivers/md/md.c

@@ -9240,9 +9240,14 @@ static bool md_spares_need_change(struct mddev *mddev)
 {
 	struct md_rdev *rdev;
 
-	rdev_for_each(rdev, mddev)
-		if (rdev_removeable(rdev) || rdev_addable(rdev))
+	rcu_read_lock();
+	rdev_for_each_rcu(rdev, mddev) {
+		if (rdev_removeable(rdev) || rdev_addable(rdev)) {
+			rcu_read_unlock();
 			return true;
+		}
+	}
+	rcu_read_unlock();
 	return false;
 }