在文章https://chronicle.software/unique-timestamp-identifiers/之后，我检查了DistributedUniqueTimeProvider.currentTimeNanos方法的实现。

long time = provider.currentTimeNanos();
    long time0 = bytes.readVolatileLong(LAST_TIME);
    long timeN = timestampFor(time) + hostId;

    if (timeN > time0 && bytes.compareAndSwapLong(LAST_TIME, time0, timeN))
        return timeN;

    return currentTimeNanosLoop();

默认情况下，它使用SystemTimeProvider
如果我们查看net.openhft.历史记录.核心.时间.系统时间提供者#当前时间纳米

long nowNS = System.nanoTime();
    long nowMS = currentTimeMillis() * NANOS_PER_MILLI;
    long estimate = nowNS + delta;

    if (estimate < nowMS) {
        delta = nowMS - nowNS;
        return nowMS;

    } else if (estimate > nowMS + NANOS_PER_MILLI) {
        nowMS += NANOS_PER_MILLI;
        delta = nowMS - nowNS;
        return nowMS;
    }
    return estimate;

我们可以看到它使用了非易失性、非原子变量

private long delta = 0;

所以问题是：DistributedUniqueTimeProvider.currentTimeNanos到底是线程安全的吗？如果是，为什么？