如何以惰性方式安全地填充 Map 线程？

Question

假设你有一个地图（编程语言无关紧要）并且想一点一点地填充它。有人可以展示如何在读写锁对象可用的情况下使这个线程安全（并行读取、独占写入）吗？

这里有一些伪代码，它不是线程安全的，可以开始使用：


def get_or_create_item(item_id)
  if (!@map.has_key?(item_id))
    @map[item_id] = create_item()
  end
  return @map[item_id]
end

def create_item
  #...
end

假设您有可用的读写锁对象，如何使其成为线程安全的？

rw_lock = ReadWriteLock.new
...
rw_lock.acquire_read()
rw_lock.release_read()
...
rw_lock.acquire_write()
rw_lock.release_write()

谢谢

Answer 1

带有 read-write 锁对象的伪代码如下所示：

ReaderWriterLock rwLock // initially unlocked

getOrCreate(id)
    // try getting
    rwLock.acquireRead()
    optional res = map[id]
    rwLock.releaseRead()
    if (res.hasValue)
        return res

    // since we are here, the item isn't in the map
    rwLock.acquireWrite()
    res = map[id]
    // check again if another thread inserted the item while we were waiting for the write lock
    if (!res.hasValue)
        res = createItem()
        map[id] = res
    rwLock.releaseWrite()
    // at this point res must have a value
    return res

如何以惰性方式安全地填充 Map 线程？

How to fill a Map thread safe in a lazy manner?

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