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(1) 原始代码
最近使用单生产者-多消费者模型是遇到一个问题,以前既然都没有想到过。生产者线程的代码如下,基本功能就是接收到一个连接之后创建一个Socket对象并放到list中等待处理。
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void DataManager::InternalStart() {
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server_socket_ = new ServerSocket();
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if (!server_socket_->SetAddress(NetworkUtil::GetIpAddress().c_str(), 9091)) {
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LOG(ERROR) << "Set address failed.";
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delete server_socket_;
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server_socket_ = NULL;
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return;
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}
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server_socket_->SetSoBlocking(true);
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if (!server_socket_->Listen()) {
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LOG(ERROR) << "listen failed.";
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return;
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}
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Socket *socket = NULL;
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while (!stop_) {
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if ((socket = server_socket_->Accept()) != NULL) {
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LOG(INFO) << "Recieved connection fd: " << socket->GetAddr();
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{
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common::MutexLock lc(&socket_mu_);
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socket_list_.push_back(socket);
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cond_var_.Signal();
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}
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}
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}
多个消费者线程的的代码如下,基本功能是从list中取得一个Socket对象进行处理;
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void DataManager::WorkEntry() {
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Socket *socket = NULL;
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while (!stop_) {
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// Get connection socket.
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{
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common::MutexLock lc(&socket_mu_);
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if (socket_list_.empty()) {
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cond_var_.Wait(&socket_mu_);
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}
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if (stop_)
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break;
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socket = socket_list_.front();
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socket_list_.pop_front();
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}
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bool success = false;
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do{
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{
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Packet request;
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if ((success = socket->GetPacket(&request))) {
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HandlePacket(&request);
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}
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}
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} while (success);
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delete socket;
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socket = NULL;
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}
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}
(2) 问题
运行过程中进场出现段错误,都是在12行(
socket = socket_list_.front())。使用GDB调试发现
socket_list_的size为0。
(3) 加入log调试
加入下面的log进行调试
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@@ -115,6 +115,7 @@ voidDataManager::InternalStart() {
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{
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common::MutexLocklc(&socket_mu_);
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socket_list_.push_back(socket);
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+ LOG(INFO) << "1:size: " << socket_list_.size();
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cond_var_.Signal();
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}
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}
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@@ -129,11 +130,14 @@ voidDataManager::WorkEntry() {
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common::MutexLock lc(&socket_mu_);
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if (socket_list_.empty()) {
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cond_var_.Wait(&socket_mu_);
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+ LOG(INFO) << "2:size: " << socket_list_.size();
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}
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if (stop_)
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break;
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+ LOG(INFO) << "3: size: " << socket_list_.size();
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socket = socket_list_.front();
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socket_list_.pop_front();
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+ LOG(INFO) << "4: size:" << socket_list_.size()
I0809 02:35:45.269896 17305DataManager.cc:114] Recieved connection fd: 10.237.92.30:37220
I0809 02:35:45.269902 17305DataManager.cc:118] 1: size: 1
I0809 02:35:45.269928 17310DataManager.cc:133] 2: size: 1
I0809 02:35:45.269935 17310DataManager.cc:137] 3: size: 1
I0809 02:35:45.269937 17310DataManager.cc:140] 4: size: 0
………
I0809 02:35:45.271636 17305 DataManager.cc:114]Recieved connection fd: 10.237.92.30:37224
I0809 02:35:45.271644 17305DataManager.cc:118] 1: size: 1
I0809 02:35:45.271663 17310DataManager.cc:137] 3: size: 1
I0809 02:35:45.271670 17310DataManager.cc:140] 4: size: 0
I0809 02:35:45.271739 17309 DataManager.cc:133]2: size: 0
I0809 02:35:45.271750 17309DataManager.cc:137] 3: size: 0
(4) 分析:
a) 正常的log顺序
正常的log顺序应该是,add一个Socket之后得到,有一个消费者线程被signal唤醒并处理这个socket。
I0809 02:35:45.269902 17305DataManager.cc:118] 1: size: 1
I0809 02:35:45.269928 17310DataManager.cc:133] 2: size: 1
I0809 02:35:45.269935 17310DataManager.cc:137] 3: size: 1
I0809 02:35:45.269937 17310DataManager.cc:140] 4: size: 0
b) 出错时的log顺序
出现错误时的log顺序如下,
I0809 02:35:45.271644 17305DataManager.cc:118] 1: size: 1
I0809 02:35:45.271663 17310DataManager.cc:137] 3: size: 1
I0809 02:35:45.271670 17310DataManager.cc:140] 4: size: 0
I0809 02:35:45.271739 17309 DataManager.cc:133]2: size: 0
I0809 02:35:45.271750 17309DataManager.cc:137] 3: size: 0
线程号可以从第三列得出, 17305的线程是生产者线程,17310和17309为消费者线程。从打印的log可以看除运行的顺序如下:
a) 初始状态;
i. 17305:获得socket_mu_准备向socket_list_中插入socket。
ii. 17309:正处于cond_var_.Wait(&socket_mu_);状态下等待cond_var发生;
iii. 17310 :socket_mu_应该是在试图
b) 17305线程调用cond_var_.Signal()唤醒17309,此时17309和17310还需要争夺socket_mu_,应该是17310先得到了socket_mu_所以17309必须再次睡眠。
c) 17310将刚才17305生产的socket消耗了,并且释放了socket_mu_。但是此时的socket_list_有变成空的了。
d) 17309得到socket_mu_,调用socket_list_.front()时程序crash。
(4) 解决办法:多加一个判断
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@@ -129,6 +129,9 @@ voidDataManager::WorkEntry() {
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common::MutexLock lc(&socket_mu_);
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if (socket_list_.empty()) {
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cond_var_.Wait(&socket_mu_);
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+
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+ if (socket_list_.empty())
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+ continue;
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}
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if (stop_)
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break
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