英特尔 TBB 程序不终止，可能滥用参考计数器

Question

我有以下 TBB 代码片段。代码未按预期工作。

#include <iostream>
#include <tbb/mutex.h>
#include <tbb/tbb.h>

using namespace std;
using namespace tbb;

tbb::mutex printLock;

class Task1 : public task {
public:
  task* execute() {
    printLock.lock();
    cout << "Task 1 start" << std::endl;
    printLock.unlock();

    printLock.lock();
    cout << "Task 1 end" << std::endl;
    printLock.unlock();

    return NULL;
  }
};

class Task2 : public task {
public:
  task* execute() {
    printLock.lock();
    cout << "Task 2 start" << std::endl;
    printLock.unlock();

    printLock.lock();
    cout << "Task 2 end" << std::endl;
    printLock.unlock();

    return NULL;
  }
};

class Task5 : public task {
public:
  task* execute() {
    printLock.lock();
    std::cout << "Task 5 start" << std::endl;
    printLock.unlock();

    printLock.lock();
    std::cout << "T5:Sleep start" << std::endl;
    printLock.unlock();
    sleep(10);
    printLock.lock();
    std::cout << "T5:Sleep end" << std::endl;
    printLock.unlock();

    printLock.lock();
    std::cout << "Task 5 end" << std::endl;
    printLock.unlock();

    return NULL;
  }
};

class Task4 : public task {
public:
  task* execute() {
    printLock.lock();
    std::cout << "Task 4 start" << std::endl;
    printLock.unlock();

    set_ref_count(1); // Create a child but do not want to wait

    task& u = *new (task::allocate_child()) Task5();
    task::spawn(u);

    // task::wait_for_all(); // Task 5 is asynchronous, just to print

    printLock.lock();
    std::cout << "Task 4 end" << std::endl;
    printLock.unlock();

    return NULL;
  }
};

class Task3 : public task {
public:
  task* execute() {
    printLock.lock();
    std::cout << "Task 3 start" << std::endl;
    printLock.unlock();

    set_ref_count(2);

    task& u = *new (task::allocate_child()) Task4();
    task::spawn(u);

    task::wait_for_all();

    printLock.lock();
    std::cout << "Task 3 end" << std::endl;
    printLock.unlock();

    return NULL;
  }
};

class Root1 : public task {
public:
  task* execute() {
    printLock.lock();
    std::cout << "Root1 start" << std::endl;
    printLock.unlock();

    set_ref_count(4);

    task& a = *new (task::allocate_child()) Task1();
    task& v = *new (task::allocate_child()) Task3();
    task& b = *new (task::allocate_child()) Task2();

    task::spawn(a);
    task::spawn(v);
    task::spawn(b);

    task::wait_for_all();

    printLock.lock();
    std::cout << "Root1 end" << std::endl;
    printLock.unlock();

    return NULL;
  }
};

int main() {
  task& v = *new (task::allocate_root()) Root1();
  task::spawn_root_and_wait(v);
  return EXIT_SUCCESS;
}

我希望看到以下（或类似）输出。任务5可能开始也可能结束，没关系。

Root 1 start 
Task 2 start
Task 2 end
Task 3 start
Task 4 start
Task 4 end
Task 3 end
Task 5 start
T5:Sleep start
Task 1 start
Task 1 end
T5:Sleep end
Task 5 end
Root 1 end

但是，代码可能会无限运行（？）并重复任务 3、4 和 5。

Root 1 start
Task 2 start
Task 2 end
Task 3 start
Task 4 start
Task 4 end
Task 3 end
Task 5 start
T5:Sleep start
Task 1 start
Task 1 end
T5:Sleep end
Task 5 end
Task 4 start
Task 4 end
Task 3 start
Task 4 start
Task 4 end
Task 5 start
T5:Sleep start
Task 5 start
T5:Sleep start
Task 5 start
T5:Sleep start
T5:Sleep end
Task 5 end
Task 4 start
Task 4 end
Task 5 start
T5:Sleep start
T5:Sleep end
T5:Sleep end
Task 5 end
Task 5 start
Task 5 end
Task 4 start
Task 4 end
Task 5 start
T5:Sleep start
Task 5 start
T5:Sleep start
T5:Sleep start
^C

我没有完全理解这个例子，这个错误可能与set_ref_count()的操作有关。任何有助于理解问题的指示都会有所帮助。谢谢

Answer 1

问题出在异步任务附近。当 Task 5 完成时，它会向 Task 4 发出信号以将其引用计数递减至零，并且 Task 4 再次运行。所以，有一个循环。因此，它会导致任务 3 的引用计数竞争，从而导致额外的循环。对于异步模式，您需要使用连续传递方法。但是，我建议使用几种 tbb::task_group 算法来简化同步问题。

英特尔 TBB 程序不终止，可能滥用参考计数器

Intel TBB program does not terminate, possible misuuse of reference counter

c++

tbb