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When running the http async_client example of the official boos asio example code in helgrind, there is a warning. This code from the file boost/asio/detail/posix_event.hpp seems to be the cause of the warning:

// Signal the event and unlock the mutex.
template <typename Lock>
void signal_and_unlock(Lock& lock)
{
    BOOST_ASSERT(lock.locked());
    signalled_ = true;
    lock.unlock();
    ::pthread_cond_signal(&cond_); // Ignore EINVAL.
}

Here is the full output of valgrind/helgrind:

jcm@Ubuntu:~/samples/async-client/build$ valgrind --tool=helgrind ./async-client stackoverflow.com error.html
==2894== Helgrind, a thread error detector
==2894== Copyright (C) 2007-2011, and GNU GPL'd, by OpenWorks LLP et al.
==2894== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==2894== Command: ./async-client stackoverflow.com error.html
==2894== 
==2894== ---Thread-Announcement------------------------------------------
==2894== 
==2894== Thread #1 is the program's root thread
==2894== 
==2894== ----------------------------------------------------------------
==2894== 
==2894== Thread #1: pthread_cond_{signal,broadcast}: dubious: associated lock is not held by any thread
==2894==    at 0x4C2C978: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==2894==    by 0x4C2E5EA: pthread_cond_signal@* (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==2894==    by 0x43FCB2: void boost::asio::detail::posix_event::signal_and_unlock<boost::asio::detail::scoped_lock<boost::asio::detail::posix_mutex> >(boost::asio::detail::scoped_lock<boost::asio::detail::posix_mutex>&) (in /home/jcm/samples/async-client/build/async-client)
==2894==    by 0x43B659: boost::asio::detail::task_io_service::wake_one_idle_thread_and_unlock(boost::asio::detail::scoped_lock<boost::asio::detail::posix_mutex>&) (in /home/jcm/samples/async-client/build/async-client)
==2894==    by 0x43B68A: boost::asio::detail::task_io_service::wake_one_thread_and_unlock(boost::asio::detail::scoped_lock<boost::asio::detail::posix_mutex>&) (in /home/jcm/samples/async-client/build/async-client)
==2894==    by 0x43B0B8: boost::asio::detail::task_io_service::post_immediate_completion(boost::asio::detail::task_io_service_operation*) (in /home/jcm/samples/async-client/build/async-client)
==2894==    by 0x43DBFD: boost::asio::detail::resolver_service_base::start_resolve_op(boost::asio::detail::task_io_service_operation*) (in /home/jcm/samples/async-client/build/async-client)
==2894==    by 0x442F86: void boost::asio::detail::resolver_service<boost::asio::ip::tcp>::async_resolve<boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> > >(std::shared_ptr<void>&, boost::asio::ip::basic_resolver_query<boost::asio::ip::tcp> const&, boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> >) (in /home/jcm/samples/async-client/build/async-client)
==2894==    by 0x44187E: void boost::asio::ip::resolver_service<boost::asio::ip::tcp>::async_resolve<boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> > >(std::shared_ptr<void>&, boost::asio::ip::basic_resolver_query<boost::asio::ip::tcp> const&, boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> >&&) (in /home/jcm/samples/async-client/build/async-client)
==2894==    by 0x44097E: void boost::asio::ip::basic_resolver<boost::asio::ip::tcp, boost::asio::ip::resolver_service<boost::asio::ip::tcp> >::async_resolve<boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> > >(boost::asio::ip::basic_resolver_query<boost::asio::ip::tcp> const&, boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> >&&) (in /home/jcm/samples/async-client/build/async-client)
==2894==    by 0x43E05F: client::client(boost::asio::io_service&, std::string const&, std::string const&) (in /home/jcm/samples/async-client/build/async-client)
==2894==    by 0x437018: main (in /home/jcm/samples/async-client/build/async-client)
==2894== 
Response returned with status code 400
==2894== 
==2894== For counts of detected and suppressed errors, rerun with: -v
==2894== Use --history-level=approx or =none to gain increased speed, at
==2894== the cost of reduced accuracy of conflicting-access information
==2894== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 89 from 29)

I seems wrong to me that mutex is unlocked before the condition variable is signalled. In case it's a false positive, please let me know why the above code is correct.

4

1 回答 1

5

假设这是匹配调用中使用的lock相同对象的包装器对象，pthreads 的大多数实现将允许您以任一顺序发出信号和解锁，因为被定义为在条件变量上解除阻塞并获取互斥锁作为一个原子操作——也就是说，它不会成功返回，直到CV已经发出信号并且调用线程已经获取了互斥锁。pthread_mutex_tpthread_cond_(timed)waitpthread_cond_(timed)wait

互联网谣言表明，无论按什么顺序执行此操作，都没有人会失败，但是一些线程调度程序的编写是为了在信号之后发生解锁时实现最高效率——而另一些线​​程调度程序是为了在信号之前发生解锁时实现最高效率而编写的，这意味着无论您如何编写它，您都无法一直获胜。就个人而言，如果我维护有问题的代码，我会仅仅因为它让 helgrind 高兴而改变顺序。您不想费力通过垃圾投诉来找到真正的比赛条件。

我在我的 POSIX API botches 列表中添加了“pthread_cond_signal应该同时使用 CV 和互斥锁，就像pthread_cond_wait当我得到一台时间机器时要修复的那样”。

于 2013-01-21T22:31:38.453 回答