那些遇到过的问题

使用packaged_task和threads进行可能的数据竞争

ste*_*fan 8 c++ valgrind pthreads packaged-task

我最近运行valgrind --tool=helgrind了我的项目并收到了一条警告"可能的数据竞争",我认为这是一个问题.但是,即使是这个简单的测试程序也会导致以下消息:

#include <iostream>
#include <thread>
#include <future>

int main()
{
   std::packaged_task<void()> task([]()
   {
      std::cout << "Hello\n"; // You can leave this out
   });
   auto future = task.get_future();
   std::thread(std::move(task)).detach();
   future.get();
}
Run Code Online (Sandbox Code Playgroud)

编译g++-4.9 -p -g -std=c++11 -pthread -O3 test.cpp,输出valgrind --tool=helgrind ./a.out是:

==12808== Helgrind, a thread error detector
==12808== Copyright (C) 2007-2013, and GNU GPL'd, by OpenWorks LLP et al.
==12808== Using Valgrind-3.10.0 and LibVEX; rerun with -h for copyright info
==12808== Command: ./a.out
==12808== 
Hello
==12808== ---Thread-Announcement------------------------------------------
==12808== 
==12808== Thread #2 was created
==12808==    at 0x567B73E: clone (clone.S:74)
==12808==    by 0x536A1F9: do_clone.constprop.4 (createthread.c:75)
==12808==    by 0x536B7EB: create_thread (createthread.c:245)
==12808==    by 0x536B7EB: pthread_create@@GLIBC_2.2.5 (pthread_create.c:606)
==12808==    by 0x4C30E3D: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x4EF7F08: std::thread::_M_start_thread(std::shared_ptr<std::thread::_Impl_base>) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==12808==    by 0x403C3D: std::thread::thread<std::packaged_task<void ()>>(std::packaged_task<void ()>&&) (thread:138)
==12808==    by 0x401D57: main (test.cpp:13)
==12808== 
==12808== ----------------------------------------------------------------
==12808== 
==12808== Thread #2: pthread_cond_{signal,broadcast}: dubious: associated lock is not held by any thread
==12808==    at 0x4C2F295: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x4EF3CE8: std::condition_variable::notify_all() (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==12808==    by 0x402F18: _M_set_result (future:374)
==12808==    by 0x402F18: _M_run (future:1319)
==12808==    by 0x402F18: operator() (future:1453)
==12808==    by 0x402F18: _M_invoke<> (functional:1700)
==12808==    by 0x402F18: operator() (functional:1688)
==12808==    by 0x402F18: std::thread::_Impl<std::_Bind_simple<std::packaged_task<void ()> ()> >::_M_run() (thread:115)
==12808==    by 0x4EF7DCF: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==12808==    by 0x4C30FD6: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x536B0A4: start_thread (pthread_create.c:309)
==12808==    by 0x567B77C: clone (clone.S:111)
==12808== 
==12808== ---Thread-Announcement------------------------------------------
==12808== 
==12808== Thread #1 is the program's root thread
==12808== 
==12808== ----------------------------------------------------------------
==12808== 
==12808== Possible data race during read of size 8 at 0x5C4FAA0 by thread #1
==12808== Locks held: none
==12808==    at 0x4026A9: ~unique_ptr (unique_ptr.h:235)
==12808==    by 0x4026A9: ~_Task_state_base (future:1281)
==12808==    by 0x4026A9: ~_Task_state (future:1302)
==12808==    by 0x4026A9: destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (new_allocator.h:124)
==12808==    by 0x4026A9: _S_destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (alloc_traits.h:282)
==12808==    by 0x4026A9: destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (alloc_traits.h:411)
==12808==    by 0x4026A9: std::_Sp_counted_ptr_inplace<std::__future_base::_Task_state<main::{lambda()#1}, std::allocator<int>, void ()>, main::{lambda()#1}, (__gnu_cxx::_Lock_policy)2>::_M_dispose() (shared_ptr_base.h:524)
==12808==    by 0x401ED4: _M_release (shared_ptr_base.h:149)
==12808==    by 0x401ED4: ~__shared_count (shared_ptr_base.h:666)
==12808==    by 0x401ED4: ~__shared_ptr (shared_ptr_base.h:914)
==12808==    by 0x401ED4: reset (shared_ptr_base.h:1015)
==12808==    by 0x401ED4: ~_Reset (future:657)
==12808==    by 0x401ED4: get (future:786)
==12808==    by 0x401ED4: main (test.cpp:14)
==12808== 
==12808== This conflicts with a previous write of size 8 by thread #2
==12808== Locks held: none
==12808==    at 0x403407: release (unique_ptr.h:328)
==12808==    by 0x403407: unique_ptr<std::__future_base::_Result<void>, std::__future_base::_Result_base::_Deleter, void> (unique_ptr.h:221)
==12808==    by 0x403407: ~packaged_task (future:1409)
==12808==    by 0x403407: ~_Head_base (tuple:128)
==12808==    by 0x403407: ~_Tuple_impl (tuple:229)
==12808==    by 0x403407: ~tuple (tuple:388)
==12808==    by 0x403407: ~_Bind_simple (functional:1663)
==12808==    by 0x403407: std::thread::_Impl<std::_Bind_simple<std::packaged_task<void ()> ()> >::~_Impl() (thread:107)
==12808==    by 0x4EF7E1A: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==12808==    by 0x4C30FD6: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x536B0A4: start_thread (pthread_create.c:309)
==12808==    by 0x567B77C: clone (clone.S:111)
==12808==  Address 0x5c4faa0 is 128 bytes inside a block of size 144 alloc'd
==12808==    at 0x4C2C520: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x401C8C: allocate (new_allocator.h:104)
==12808==    by 0x401C8C: allocate (alloc_traits.h:357)
==12808==    by 0x401C8C: __shared_count<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()>, std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr_base.h:616)
==12808==    by 0x401C8C: __shared_ptr<std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr_base.h:1090)
==12808==    by 0x401C8C: shared_ptr<std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr.h:316)
==12808==    by 0x401C8C: allocate_shared<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()>, std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr.h:588)
==12808==    by 0x401C8C: __create_task_state<void(), main()::<lambda()>, std::allocator<int> > (future:1351)
==12808==    by 0x401C8C: packaged_task<main()::<lambda()>, std::allocator<int>, void> (future:1403)
==12808==    by 0x401C8C: packaged_task<main()::<lambda()>, void> (future:1393)
==12808==    by 0x401C8C: main (test.cpp:11)
==12808==  Block was alloc'd by thread #1
==12808== 
==12808== ----------------------------------------------------------------
==12808== 
==12808== Possible data race during read of size 1 at 0x5C4FA6C by thread #1
==12808== Locks held: none
==12808==    at 0x4C2ED90: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x4C2F57F: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x4026D7: ~_State_baseV2 (future:316)
==12808==    by 0x4026D7: ~_Task_state_base (future:1281)
==12808==    by 0x4026D7: ~_Task_state (future:1302)
==12808==    by 0x4026D7: destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (new_allocator.h:124)
==12808==    by 0x4026D7: _S_destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (alloc_traits.h:282)
==12808==    by 0x4026D7: destroy<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()> > (alloc_traits.h:411)
==12808==    by 0x4026D7: std::_Sp_counted_ptr_inplace<std::__future_base::_Task_state<main::{lambda()#1}, std::allocator<int>, void ()>, main::{lambda()#1}, (__gnu_cxx::_Lock_policy)2>::_M_dispose() (shared_ptr_base.h:524)
==12808==    by 0x401ED4: _M_release (shared_ptr_base.h:149)
==12808==    by 0x401ED4: ~__shared_count (shared_ptr_base.h:666)
==12808==    by 0x401ED4: ~__shared_ptr (shared_ptr_base.h:914)
==12808==    by 0x401ED4: reset (shared_ptr_base.h:1015)
==12808==    by 0x401ED4: ~_Reset (future:657)
==12808==    by 0x401ED4: get (future:786)
==12808==    by 0x401ED4: main (test.cpp:14)
==12808== 
==12808== This conflicts with a previous write of size 4 by thread #2
==12808== Locks held: none
==12808==    at 0x536F9BF: pthread_cond_broadcast@@GLIBC_2.3.2 (pthread_cond_broadcast.S:59)
==12808==    by 0x402F18: _M_set_result (future:374)
==12808==    by 0x402F18: _M_run (future:1319)
==12808==    by 0x402F18: operator() (future:1453)
==12808==    by 0x402F18: _M_invoke<> (functional:1700)
==12808==    by 0x402F18: operator() (functional:1688)
==12808==    by 0x402F18: std::thread::_Impl<std::_Bind_simple<std::packaged_task<void ()> ()> >::_M_run() (thread:115)
==12808==  Address 0x5c4fa6c is 76 bytes inside a block of size 144 alloc'd
==12808==    at 0x4C2C520: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==12808==    by 0x401C8C: allocate (new_allocator.h:104)
==12808==    by 0x401C8C: allocate (alloc_traits.h:357)
==12808==    by 0x401C8C: __shared_count<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()>, std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr_base.h:616)
==12808==    by 0x401C8C: __shared_ptr<std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr_base.h:1090)
==12808==    by 0x401C8C: shared_ptr<std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr.h:316)
==12808==    by 0x401C8C: allocate_shared<std::__future_base::_Task_state<main()::<lambda()>, std::allocator<int>, void()>, std::allocator<int>, main()::<lambda()>, const std::allocator<int>&> (shared_ptr.h:588)
==12808==    by 0x401C8C: __create_task_state<void(), main()::<lambda()>, std::allocator<int> > (future:1351)
==12808==    by 0x401C8C: packaged_task<main()::<lambda()>, std::allocator<int>, void> (future:1403)
==12808==    by 0x401C8C: packaged_task<main()::<lambda()>, void> (future:1393)
==12808==    by 0x401C8C: main (test.cpp:11)
==12808==  Block was alloc'd by thread #1
==12808== 
==12808== 
==12808== For counts of detected and suppressed errors, rerun with: -v
==12808== Use --history-level=approx or =none to gain increased speed, at
==12808== the cost of reduced accuracy of conflicting-access information
==12808== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 23 from 23)
Run Code Online (Sandbox Code Playgroud)

我的问题:我可以安全地忽略这一点吗？如果是这样,如何抑制这些警告？

Arn*_*gel 1

我也遇到了这个警告，并检查了 Kubuntu 14.04 上 GCC 4.8 ( ) 的 libstdc++ 源代码/usr/include/c++/4.8/future。我没有发现任何错误。的future状态除其他外还包含结果指针、互斥体和条件变量。future<T>::wait()锁定互斥体并检查结果指针是否为空。如果是，它会等待条件变量（这也会在一个原子操作中解锁互斥体）。在promise<T>::set_value()（或者set_exception()，我猜）上，代码锁定互斥体，设置结果指针，解锁互斥体，然后notify_all()调用condition variable. notify_all()因此 Helgrind 会发出警告，因为调用时互斥体未锁定。然而，代码仍然是安全的，因为互斥量是为了设置指针而保存的，这是更改检查结果的唯一方法future<T>::wait()。在 的生命周期之外进行通知lock_guard可能是一种优化（尽可能短暂地保留互斥体）。不幸的是，它确实导致了这个警告。libstdc++ 维护者可能不愿意接受补丁，因为 a) 补丁会降低性能（如果轻微的话），b) 如果补丁没有损坏，则不要修复它。因此我的建议是压制赫尔格瑞德。

请注意，为了清晰和简洁，我简化了一些交互，特别future是promise将实际工作委托给隐藏的实现类，set_value使用附加功能once_flag来同步并发调用等。

另请参阅此线程、我对它的回答以及 Cubbi 对我的答案的精确评论（我忽略了这一点）： Why do those the notify and wait function of a std::condition_variable need a Locked mutex

归档时间：

查看次数：

547 次

最近记录：

10 年，8 月 前
为什么std :: condition_variable的notify和wait函数都需要一个锁定的互斥锁 5
更多相关链接
相关归档
浮点与现代硬件上的整数计算 96
如何声明一个接受lambda的函数？ 75
为什么编译器不允许std :: string在union中？ 45
使用C++ 11,编写f(x ++),g(x ++)是不确定的行为？ 42
如何在C++中初始化3D数组 35
c ++ find_if lambda 35
我是否必须在客户端程序中绑定UDP套接字才能接收数据？(我总是得到WSAEINVAL) 28
如何折叠STL容器？ 26
C：分段错误：GDB：&lt;错误读取变量&gt; 6
获取 malloc 合并错误的示例程序 5
难疑归档
堆栈和堆的内容和位置是什么？ 7847
Python有三元条件运算符吗？ 5591
如何检查文件是否存在而没有例外？ 5290
如何从SQL Server中的SELECT更新？ 3546
如何删除子模块？ 3366
如何在Python中将字符串解析为float或int？ 2108
什么是反思,为什么它有用？ 2011
CSS三角形如何工作？ 1791
如果我已经开始使用rebase,如何将两个提交合并为一个？ 1111
功能编程是否取代了GoF设计模式？ 1028