如果这不是 boost::lockfree::detail::freelist 中的错误，我在这里缺少什么？

Question

在this file, the boost::lockfree::detail::freelist class is used to manage storage for a lock-free data structure (e.g., a queue), using a free list。 deallocate_impl 方法用于通过将节点链接回空闲列表来释放节点（已释放的节点成为空闲列表的新头，取代旧头）。此方法应该是线程安全且无锁的。此处复制了一个实例的原始源代码，并注释了我的注释以指出可疑代码（潜在错误？）：

void deallocate_impl (index_t index)
{
    freelist_node * new_pool_node =
           reinterpret_cast<freelist_node*>(NodeStorage::nodes() + index);

    // Shouldn't this line be placed inside the loop?
    // The loop continues as long as the compare-and-exchange fails,
    // which happens if the pool head has been concurrently updated.
    // In that case, we MUST reload the new value of "pool_" to
    // reuse its tag, link the new free-list head to it and
    // compare against in the compare_and_exchange call.
    tagged_index old_pool = pool_.load(memory_order_consume);

    for(;;) {
        // The old pool head tag is reused here without causing
        // any ABA problems. However, if "index" is the same as
        // old_pool.get_index(), a self-reference is written.
        tagged_index new_pool (index, old_pool.get_tag());
        new_pool_node->next.set_index(old_pool.get_index());

        if (pool_.compare_exchange_weak(old_pool, new_pool))
            return;
    }
}

我在 Boost 1.62.0 中也看到了相同的实现

Answer 1

The loop continues as long as the compare-and-exchange fails, which happens if the pool head has been concurrently updated. In that case, we MUST reload the new value of "pool_" to reuse its tag...

compare_exchange_weak() 在每次调用后将 pool_ 的先前值写入 old_pool。 Documentation for compare_exchange_weak().

However, if "index" is the same as old_pool.get_index()...

这可能不会发生，因为具有该索引的节点尚未移至空闲列表。

如果这不是 boost::lockfree::detail::freelist 中的错误，我在这里缺少什么？

If this is not a bug in boost::lockfree::detail::freelist, what am I missing here?

c++

multithreading

boost

queue

lock-free