Java-关于重新进入的奇怪并发问题

Question

我有2个测试用例，而第一个案例失败了，第二个可以成功。 唯一的差异代码是类cons engry 的earm 。 为什么？

public class ReentrantSpinLockTest {
    private final Incr incr = new Incr();

    @Test
    public void lock() throws InterruptedException {
        int n = 50;
        int inner_loop = 100_000;
        CountDownLatch boot = new CountDownLatch(n);
        CountDownLatch complete = new CountDownLatch(n);
        for (int i = 0; i < n; ++i) {
            new Thread(() -> {
                try {
                    boot.await();
                    for (int k = 0; k < inner_loop; ++k) {
                        incr.incr();
                    }
                } catch (InterruptedException e) {
                    throw new RuntimeException(e);
                } finally {
                    complete.countDown();
                }
            }).start();
            boot.countDown();
        }
        complete.await();
        int expect = n * inner_loop;
        int actual = incr.getI();
        assert expect == actual : String.format("expect %d actual %d", expect, actual);
    }

}

成功的递增：

public class Incr {
        private final ReentrantLock lock = new ReentrantLock();
        private volatile int i;

        // only this diff
        public void incr() {
            lock.lock();
            lock.lock();
            i++;
            lock.unlock();
            lock.unlock();
        }

        public int getI() {
            return i;
        }
    }

失败的递增：

    public class Incr {
        private final ReentrantLock lock = new ReentrantLock();
        private volatile int i;

        public void incr() {
            int k = Math.abs(new Random().nextInt() % 5);
            for (int t = 0; t < k; ++t) {
                lock.lock();
            }
            i++;
            for (int t = 0; t < k; ++t) {
                lock.unlock();
            }
        }

        public int getI() {
            return i;
        }
    }