如果这不是 boost::lockfree::d etail::freelist 中的错误，我在这里错过了什么

Question

在此文件中，boost::lockfree::detail::freelist 类用于使用空闲列表管理无锁数据结构(例如队列(的存储。deallocate_impl 方法用于通过将节点链接回空闲列表来释放节点(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。一个实例的原始源代码在这里重复，并注释了我的评论以指出可疑代码(潜在的错误？

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;
    }
}

我也在 Boost 1.62.0 中看到过相同的实现

Answer 1

只要比较和交换失败，循环就会继续，如果池头已同时更新，就会发生这种情况。在这种情况下，我们必须重新加载"pool_"的新值以重用其标签......

compare_exchange_weak()在每次调用后将以前的 pool_ 值写入old_pool。compare_exchange_weak()文档 .

但是，如果"index"与old_pool.get_index((相同...

这可能不会发生，因为具有该索引的节点尚未移动到自由列表。