使用 helgrind 提升 asio http async_client 示例警告：误报？

Question

在helgrind中运行官方boos asio示例代码的http async_client示例时，出现警告。文件中的这段代码boost/asio/detail/posix_event.hpp似乎是警告的原因：

// 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.
}

这是 valgrind/helgrind 的完整输出：

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)

在我看来，互斥锁在条件变量发出信号之前被解锁是错误的。如果是误报，请告诉我为什么上面的代码是正确的。

Answer 1

假设这lock是pthread_mutex_t匹配调用中使用的相同对象的包装对象pthread_cond_(timed)wait，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pthread_cond_(timed)wait被定义为在条件变量上解除阻塞并获取互斥体的原子操作-也就是说，它不会成功返回，直到双方的CV已经发出信号，并调用线程已经获取互斥。

互联网谣言表明，无论它的顺序如何，没有人会失败此操作，但是当解锁发生在信号之后时，一些线程调度程序已被编写以实现最高效率——而当解锁发生在信号之前发生时，其他线程调度程序已被编写以实现最高效率，这意味着无论你怎么写，你都不能一直赢。就我个人而言，如果我要维护有问题的代码，我会改变顺序，因为它能让 helgrind 高兴。您不想费力地通过垃圾投诉来找到真正的比赛条件。

我正在添加“pthread_cond_signal应该同时使用 CV 和互斥锁，就像pthread_cond_wait”到我的 POSIX API 漏洞列表中，以便在我获得时间机器时进行修复。