如何避免与Helgrind的误报？

2 回答

• 1

  问题是Helgrind并不了解GCC的原子内置，所以没有意识到它们是无竞争的并且对程序进行了排序 .

  有一些方法可以注释你的代码以帮助Helgrind，请参阅http://valgrind.org/docs/manual/hg-manual.html#hg-manual.effective-use（但我不知道如何在这里使用它们，我已经尝试了sbabbi显示的内容，它只解决了部分问题） .

  无论如何，我会避免在繁忙的循环中屈服，这是一种糟糕的同步形式 . 可以使用如下的条件变量来完成：

  #include <thread>
  #include <atomic>
  #include <iostream>
  #include <condition_variable>
  
  int main()
  {
      bool isReady(false);
      std::mutex mx;
      std::condition_variable cv;
  
      int i = 1;
  
      std::thread t([&isReady, &i, &mx, &cv]()
      {
          i = 2;
          std::unique_lock<std::mutex> lock(mx);
          isReady = true;
          cv.notify_one();
      });
  
      {
          std::unique_lock<std::mutex> lock(mx);
          cv.wait(lock, [&] { return isReady; });
      }
  
      i = 3;
  
      t.join();
  
      std::cout << i;
  
      return 0;
  }
  

  回复于 2024-05-05T04:55:06+08:00
• 7

  Valgrind无法知道 while (!isReady) 循环（以及存储和加载上的隐式 memory_order_release 和 memory_order_consume 标志），意味着 i = 2 语句是 i = 3 之前的依赖顺序 .

  您必须使用valgrind ANNOTATE_HAPPENS_BEFORE 和 ANNOTATE_HAPPENS_AFTER 宏显式声明此不变量：

  #include <valgrind/drd.h>
  
  #include <thread>
  #include <atomic>
  #include <iostream>
  
  int main()
  {
      std::atomic<bool> isReady(false);
  
      int i = 1;
  
      std::thread t([&isReady, &i]()
      {
          i = 2;
          ANNOTATE_HAPPENS_BEFORE(&isReady);
          isReady = true;
      });
  
      while (!isReady)
          std::this_thread::yield();
  
      ANNOTATE_HAPPENS_AFTER(&isReady);
      i = 3;
  
      t.join();
  
      std::cout << i;
  
      return 0;
  }
  

  这里我们说 ANNOTATE_HAPPENS_BEFORE 的行始终发生在 ANNOTATE_HAPPENS_AFTER 行之前，我们知道由于检查程序逻辑，但是valgrind无法为你证明 .

  该计划产生：

  valgrind --tool=helgrind ./a.out
  ==714== Helgrind, a thread error detector
  ==714== Copyright (C) 2007-2015, and GNU GPL'd, by OpenWorks LLP et al.
  ==714== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info
  ==714== Command: ./val
  ==714== 
  ==714== ---Thread-Announcement------------------------------------------
  ==714== 
  ==714== Thread #1 is the program's root thread
  ==714== 
  ==714== ---Thread-Announcement------------------------------------------
  ==714== 
  ==714== Thread #2 was created
  ==714==    at 0x59E169E: clone (in /usr/lib/libc-2.23.so)
  ==714==    by 0x4E421D9: create_thread (in /usr/lib/libpthread-2.23.so)
  ==714==    by 0x4E43C42: pthread_create@@GLIBC_2.2.5 (in /usr/lib/libpthread-2.23.so)
  ==714==    by 0x4C316F3: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
  ==714==    by 0x4C327D7: pthread_create@* (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
  ==714==    by 0x5113DB4: __gthread_create (gthr-default.h:662)
  ==714==    by 0x5113DB4: std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State> >, void (*)()) (thread.cc:163)
  ==714==    by 0x40109C: std::thread::thread<main::{lambda()#1}>(main::{lambda()#1}&&) (in /home/ennio/val)
  ==714==    by 0x400F55: main (in /home/ennio/val)
  ==714== 
  ==714== ----------------------------------------------------------------
  ==714== 
  ==714== Possible data race during read of size 1 at 0xFFF00061F by thread #1
  ==714== Locks held: none
  ==714==    at 0x401585: std::atomic<bool>::operator bool() const (in /home/ennio/val)
  ==714==    by 0x400F61: main (in /home/ennio/val)
  ==714== 
  ==714== This conflicts with a previous write of size 1 by thread #2
  ==714== Locks held: none
  ==714==    at 0x4015D5: std::__atomic_base<bool>::operator=(bool) (in /home/ennio/val)
  ==714==    by 0x401550: std::atomic<bool>::operator=(bool) (in /home/ennio/val)
  ==714==    by 0x400F1B: main::{lambda()#1}::operator()() const (in /home/ennio/val)
  ==714==    by 0x40146F: void std::_Bind_simple<main::{lambda()#1} ()>::_M_invoke<>(std::_Index_tuple<>) (in /home/ennio/val)
  ==714==    by 0x40140C: std::_Bind_simple<main::{lambda()#1} ()>::operator()() (in /home/ennio/val)
  ==714==    by 0x4013EB: std::thread::_State_impl<std::_Bind_simple<main::{lambda()#1} ()> >::_M_run() (in /home/ennio/val)
  ==714==    by 0x5113A9E: execute_native_thread_routine (thread.cc:83)
  ==714==    by 0x4C318E7: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
  ==714==  Address 0xfff00061f is on thread #1's stack
  ==714==  in frame #1, created by main (???:)
  ==714== 
  3==714== 
  ==714== For counts of detected and suppressed errors, rerun with: -v
  ==714== Use --history-level=approx or =none to gain increased speed, at
  ==714== the cost of reduced accuracy of conflicting-access information
  ==714== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
  

  要删除 isReady 本身的错误，我假设 __atomic_base<bool>::operator= 上的抑制文件就足够了 .

  回复于 2024-05-05T04:55:06+08:00