folly无锁队列，尝试添加新的函数（续）

基于上一篇文章，dropHead取出节点后，删除节点，会出现内存访问的问题。按照这个逻辑，如果将移出的节点保存到一个无锁队列中，然后在需要节点的时候，从这个备用的无锁队列中取出节点，那么应该就可以避开之前的问题，现在重要的是，判断在程序运行

过程中，备用的琐碎队列的大致长度，会不会需要耗费很多的资源。

下面为修改后的folly代码：

/*
* Copyright 2014-present Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*   http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

#pragma once

#include <atomic>
#include <cassert>
#include <utility>

namespace folly {

    /**
    * A very simple atomic single-linked list primitive.
    *
    * Usage:
    *
    * class MyClass {
    *   AtomicIntrusiveLinkedListHook<MyClass> hook_;
    * }
    *
    * AtomicIntrusiveLinkedList<MyClass, &MyClass::hook_> list;
    * list.insert(&a);
    * list.sweep([] (MyClass* c) { doSomething(c); }
    */
    template <class T>
    struct AtomicIntrusiveLinkedListHook {
        T* next{ nullptr };
    };

    template <class T, AtomicIntrusiveLinkedListHook<T> T::*HookMember>
    class AtomicIntrusiveLinkedList {
    public:
        AtomicIntrusiveLinkedList() {}
        AtomicIntrusiveLinkedList(const AtomicIntrusiveLinkedList&) = delete;
        AtomicIntrusiveLinkedList& operator=(const AtomicIntrusiveLinkedList&) =
            delete;
        AtomicIntrusiveLinkedList(AtomicIntrusiveLinkedList&& other) noexcept {
            auto tmp = other.head_.load();
            other.head_ = head_.load();
            head_ = tmp;
        }
        AtomicIntrusiveLinkedList& operator=(
            AtomicIntrusiveLinkedList&& other) noexcept {
            auto tmp = other.head_.load();
            other.head_ = head_.load();
            head_ = tmp;

            return *this;
        }

        /**
        * Note: list must be empty on destruction.
        */
        ~AtomicIntrusiveLinkedList() {
            assert(empty());
        }

        bool empty() const {
            return head_.load() == nullptr;
        }

        /**
        * Atomically insert t at the head of the list.
        * @return True if the inserted element is the only one in the list
        *         after the call.
        */
        bool insertHead(T* t) {
            assert(next(t) == nullptr);

            auto oldHead = head_.load(std::memory_order_relaxed);
            do {
                next(t) = oldHead;
                /* oldHead is updated by the call below.
                NOTE: we don't use next(t) instead of oldHead directly due to
                compiler bugs (GCC prior to 4.8.3 (bug 60272), clang (bug 18899),
                MSVC (bug 819819); source:
                http://en.cppreference.com/w/cpp/atomic/atomic/compare_exchange */
            } while (!head_.compare_exchange_weak(oldHead, t,
                std::memory_order_release,
                std::memory_order_relaxed));

            return oldHead == nullptr;
        }

        /**
        * Replaces the head with nullptr,
        * and calls func() on the removed elements in the order from tail to head.
        * Returns false if the list was empty.
        */
        template <typename F>
        bool sweepOnce(F&& func) {
            if (auto head = head_.exchange(nullptr)) {
                auto rhead = reverse(head);
                unlinkAll(rhead, std::forward<F>(func));
                return true;
            }
            return false;
        }

        // new function
        // if std::memory_order_acquire applies to next(oldHead)(the first one, the argument of compare_exchange_weak)
        // and I don't know if following bugs affect the code
        // GCC prior to 4.8.3 (bug 60272), clang prior to 2014-05-05 (bug 18899)
        // MSVC prior to 2014-03-17 (bug 819819). 
        // template <typename F>
        T* sweepHead()
        {
            // handle if the list is not empty
            auto oldHead = head_.load(std::memory_order_relaxed);

            while (oldHead != nullptr && !head_.compare_exchange_weak(oldHead, next(oldHead), std::memory_order_acquire, std::memory_order_relaxed))
                ;
            // if drop out head successfully
            if (oldHead)
            {
                next(oldHead) = nullptr;
                return oldHead;
            }

            return nullptr;
        }

        // new function
        // if std::memory_order_acquire does not apply to next(oldHead)
        // and I don't know if following bugs affect the code
        // GCC prior to 4.8.3 (bug 60272), clang prior to 2014-05-05 (bug 18899)
        // MSVC prior to 2014-03-17 (bug 819819). 
        //template <typename F>
        T* dropHead()
        {
            T* oldHead = nullptr;
            // handle if the list is not empty
            while ((oldHead = head_.load(std::memory_order_acquire)))
            {
                assert(oldHead != nullptr);
                T* nextHead = next(oldHead);
                // because insert and drop out will be involving with head_, they 
                // will change head_ first, then others
                bool res = head_.compare_exchange_weak(oldHead, nextHead, std::memory_order_relaxed,
                    std::memory_order_relaxed);
                if (res && oldHead != nullptr)
                {
                    assert(next(oldHead) == nextHead);
                    next(oldHead) = nullptr;
                    return oldHead;
                }
            }

            return nullptr;
        }

        /**
        * Repeatedly replaces the head with nullptr,
        * and calls func() on the removed elements in the order from tail to head.
        * Stops when the list is empty.
        */
        template <typename F>
        void sweep(F&& func) {
            while (sweepOnce(func)) {
            }
        }

        /**
        * Similar to sweep() but calls func() on elements in LIFO order.
        *
        * func() is called for all elements in the list at the moment
        * reverseSweep() is called.  Unlike sweep() it does not loop to ensure the
        * list is empty at some point after the last invocation.  This way callers
        * can reason about the ordering: elements inserted since the last call to
        * reverseSweep() will be provided in LIFO order.
        *
        * Example: if elements are inserted in the order 1-2-3, the callback is
        * invoked 3-2-1.  If the callback moves elements onto a stack, popping off
        * the stack will produce the original insertion order 1-2-3.
        */
        template <typename F>
        void reverseSweep(F&& func) {
            // We don't loop like sweep() does because the overall order of callbacks
            // would be strand-wise LIFO which is meaningless to callers.
            auto head = head_.exchange(nullptr);
            unlinkAll(head, std::forward<F>(func));
        }

    private:
        std::atomic<T*> head_{ nullptr };

        static T*& next(T* t) {
            return (t->*HookMember).next;
        }

        /* Reverses a linked list, returning the pointer to the new head
        (old tail) */
        static T* reverse(T* head) {
            T* rhead = nullptr;
            while (head != nullptr) {
                auto t = head;
                head = next(t);
                next(t) = rhead;
                rhead = t;
            }
            return rhead;
        }

        /* Unlinks all elements in the linked list fragment pointed to by `head',
        * calling func() on every element */
        template <typename F>
        void unlinkAll(T* head, F&& func) {
            while (head != nullptr) {
                auto t = head;
                head = next(t);
                next(t) = nullptr;
                func(t);
            }
        }
    };

} // namespace folly

下面是测试使用的代码：

#include <memory>
#include <cassert>

#include <iostream>
#include <vector>
#include <thread>
#include <future>
#include <random>
#include <cmath>

#include "folly.h"

using namespace folly;

struct student_name
{
    student_name(int age = 0)
        : age(age)
    {

    }

    int age;
    AtomicIntrusiveLinkedListHook<student_name> node;
};

using ATOMIC_STUDENT_LIST = AtomicIntrusiveLinkedList<student_name, &student_name::node>;

ATOMIC_STUDENT_LIST g_students;
ATOMIC_STUDENT_LIST g_backStudents;

// 统计backStudents的大小
int g_backSize = 0;

std::atomic<int> g_inserts; // insert num (successful)
std::atomic<int> g_drops;   // drop num (successful)

std::atomic<int> g_printNum;    // as same as g_drops

std::atomic<long> g_ageInSum;   // age sum when producing student_name
std::atomic<long> g_ageOutSum;  // age sum when consuming student_name

constexpr int HANDLE_NUM = 2000000;    // when testing, no more than this number, you know 20,000,000 * 100 ~= MAX_INT

constexpr int PRODUCE_THREAD_NUM = 3;     // producing thread number
constexpr int CONSUME_THREAD_NUM = 3;     // consuming thread number

inline void printOne(student_name* t)
{
    g_printNum.fetch_add(1, std::memory_order_relaxed);
    g_ageOutSum.fetch_add(t->age, std::memory_order_relaxed);
    // clean node
    // delete t;
    g_backStudents.insertHead(t);
}

void eraseOne(student_name* t)
{
    ++g_backSize;
    delete t;
}

void insert_students(int idNo)
{
    std::default_random_engine dre(time(nullptr));
    std::uniform_int_distribution<int> ageDi(1, 99);

    while (true)
    {
        int newAge = ageDi(dre);
        g_ageInSum.fetch_add(newAge, std::memory_order_relaxed);
        auto ns = g_backStudents.dropHead();
        if (ns == nullptr)
        {
            ns = new student_name(newAge);
        }

        g_students.insertHead(ns);
        // use memory_order_relaxed avoiding affect folly memory order
        g_inserts.fetch_add(1, std::memory_order_relaxed);

        // use memory_order_relaxed avoiding affect folly memory order
        if (g_inserts.load(std::memory_order_relaxed) >= HANDLE_NUM)
        {
            return;
        }
    }
}

void drop_students(int idNo)
{
    while (true)
    {
        auto st = g_students.dropHead();
        if (st)
        {
            printOne(st);
            // use memory_order_relaxed avoiding affect folly memory order
            g_drops.fetch_add(1, std::memory_order_relaxed);
        }

        // use memory_order_relaxed avoiding affect folly memory order
        if (g_drops.load(std::memory_order_relaxed) >= HANDLE_NUM)
        {
            return;
        }
    }
}

int main()
{
    std::vector<std::future<void>> insert_threads;
    for (int i = 0; i != PRODUCE_THREAD_NUM; ++i)
    {
        insert_threads.push_back(std::async(std::launch::async, insert_students, i));
    }

    std::vector<std::future<void>> drop_threads;
    for (int i = 0; i != CONSUME_THREAD_NUM; ++i)
    {
        drop_threads.push_back(std::async(std::launch::async, drop_students, i));
    }

    for (auto& item : insert_threads)
    {
        item.get();
    }

    for (auto& item : drop_threads)
    {
        item.get();
    }

    std::cout << "insert count1: " << g_inserts.load() << std::endl;
    std::cout << "drop count1: " << g_drops.load() << std::endl;
    std::cout << "print num1: " << g_printNum.load() << std::endl;

    std::cout << "age in1: " << g_ageInSum.load() << std::endl;
    std::cout << "age out1: " << g_ageOutSum.load() << std::endl;

    std::cout << std::endl;

    while (true)
    {
        auto st = g_students.dropHead();
        if (st)
        {
            printOne(st);
            // use memory_order_relaxed avoiding affect folly memory order
            g_drops.fetch_add(1, std::memory_order_relaxed);
        }

        if (g_students.empty())
        {
            break;
        }
    }

    std::cout << "insert count2: " << g_inserts.load() << std::endl;
    std::cout << "drop count2: " << g_drops.load() << std::endl;
    std::cout << "print num2: " << g_printNum.load() << std::endl;

    std::cout << "age in2: " << g_ageInSum.load() << std::endl;
    std::cout << "age out2: " << g_ageOutSum.load() << std::endl;

    g_backStudents.sweepOnce(eraseOne);

    std::cout << "back Students size: " << g_backSize << std::endl;
}

测试结果显示：

在folly.h文件中，dropHead函数的断言 assert(next(oldHead) == nextHead); 会触发，这个问题让我感到很意外，经过我认真思考，我发现了其中可能出现的问题。

说明如下：

现在假设有两个获取g_students节点的线程（调用drop_students函数），两者同时运行到获取nextHead（参考dropHead函数），然后其中一个线程（线程A）中断，另外一个线程（线程B）获取了节点（节点a，节点a的next指向节点b），这个节点被插入到g_backStudents中，这时线程B从g_students中再取出一个节点（节点b，节点b的next指向节点c），然后向g_students中插入节点的线程（调用insert_students函数）（线程C）将节点a插入到g_students中，这时，线程A继续运行，运行head_.compare_exchange_weak函数后，则head_指向节点b，而实际上此时的head_应该指向节点c，当前情况下，有两个节点指向了节点b，程序会出现问题。

当然，我所描述的只是出现问题的一种情况，实际上可能会有很多类似的情况，在这里就不一一举例，但是对于更多线程的情况，显然上面描述的情况是合理的，因为只要假设新增加的线程在上述过程中都处于中断状态就可以了。另外，在更多线程的时候，可能会有更多种出现问题的情况，在这里，我只是为了说明上述实现的不合理性。在上一篇，第一条评论中描述的问题，也可以做类似分析，只是将插入到g_backStudents改为delete，将从g_backStudents中获取节点，改为又在delete的地址创建了一个新的节点（虽然可能性很小，但是这种可能性是存在的）。

在这里，我只是展示一种错误的情况，上述的问题，如果将next节点改为shared_ptr，那么在C++20的编译环境下，或许能够解决，不过，这种修改带来的性能损耗，内存占用增加，与使用无锁队列的本意相违背，这种情况下，将原子操作改为自旋锁，说不定更好。

所以我暂时没有继续尝试下去，有兴趣的人可以考虑，如果有什么好的发现，希望能够分享一下。