Helgrind 在简单的 boost::asio::thread_pool 程序中报告同步错误

Question

我正在试验 boost::asio::thread_pool 和 helgrind 在一个具有空任务功能的简单程序中报告错误。问题出在哪里，我该如何解决？

#include <boost/thread/mutex.hpp>
#include <boost/thread.hpp>
#include <boost/asio/thread_pool.hpp>
#include <boost/asio/post.hpp>


int main() {
    ushort thread_num = 4;
    boost::asio::thread_pool pool(thread_num);

    auto task = []() {};

    for (ushort i = 0; i < thread_num; ++i)
        boost::asio::post(pool, task);

    pool.join();

    return 0;
}

这里是 helmgrind 的输出：

==266706== Thread #1 is the program's root thread
==266706== 
==266706== ----------------------------------------------------------------
==266706== 
==266706== Thread #1: pthread_cond_{signal,broadcast}: dubious: associated lock is not held by any thread
==266706==    at 0x48405D6: ??? (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_helgrind-amd64-linux.so)
==266706==    by 0x11508D: bool boost::asio::detail::posix_event::maybe_unlock_and_signal_one<boost::asio::detail::conditionally_enabled_mutex::scoped_lock>(boost::asio::detail::conditionally_enabled_mutex::scoped_lock&) (in /home/arno/Programming/test/a.out)
==266706==    by 0x111AAD: boost::asio::detail::conditionally_enabled_event::maybe_unlock_and_signal_one(boost::asio::detail::conditionally_enabled_mutex::scoped_lock&) (in /home/arno/Programming/test/a.out)
==266706==    by 0x11361E: boost::asio::detail::scheduler::wake_one_thread_and_unlock(boost::asio::detail::conditionally_enabled_mutex::scoped_lock&) (in /home/arno/Programming/test/a.out)
==266706==    by 0x1132C4: boost::asio::detail::scheduler::post_immediate_completion(boost::asio::detail::scheduler_operation*, bool) (in /home/arno/Programming/test/a.out)
==266706==    by 0x10DD99: void boost::asio::thread_pool::executor_type::post<boost::asio::detail::work_dispatcher<main::{lambda()#1}>, std::allocator<void> >(boost::asio::detail::work_dispatcher<main::{lambda()#1}>&&, std::allocator<void> const&) const (in /home/arno/Programming/test/a.out)
==266706==    by 0x10DBFB: void boost::asio::detail::initiate_post::operator()<main::{lambda()#1}&, boost::asio::thread_pool::executor_type const&>(main::{lambda()#1}&, boost::asio::thread_pool::executor_type const&) const (in /home/arno/Programming/test/a.out)
==266706==    by 0x10DB82: void boost::asio::async_result<main::{lambda()#1}, void ()>::initiate<boost::asio::detail::initiate_post, {lambda()#1}&, boost::asio::thread_pool::executor_type const&>(boost::asio::detail::initiate_post&&, {lambda()#1}&, boost::asio::thread_pool::executor_type const&) (in /home/arno/Programming/test/a.out)
==266706==    by 0x10DB54: std::enable_if<void ()::async_result_has_initiate_memfn<main::{lambda()#1}&, void ()>::value, boost::asio::async_result<std::decay<void ()::async_result_has_initiate_memfn>::type, main::{lambda()#1}&>::return_type>::type boost::asio::async_initiate<main::{lambda()#1}&, void (), boost::asio::detail::initiate_post, boost::asio::thread_pool::executor_type const&>(boost::asio::detail::initiate_post&&, void (&)()::async_result_has_initiate_memfn, boost::asio::thread_pool::executor_type const&) (in /home/arno/Programming/test/a.out)
==266706==    by 0x10DB12: boost::asio::async_result<std::decay<main::{lambda()#1}&>::type, void ()>::return_type boost::asio::post<boost::asio::thread_pool::executor_type, main::{lambda()#1}&>(boost::asio::thread_pool::executor_type const&, std::decay&&, std::enable_if<boost::asio::is_executor<boost::asio::async_result<std::decay<main::{lambda()#1}&>::type, void ()>::return_type>::value, void>::type*) (in /home/arno/Programming/test/a.out)
==266706==    by 0x10DABD: boost::asio::async_result<std::decay<main::{lambda()#1}&>::type, void ()>::return_type boost::asio::post<boost::asio::thread_pool, main::{lambda()#1}&>(boost::asio::thread_pool&, std::decay&&, std::enable_if<std::is_convertible<boost::asio::thread_pool, boost::asio::execution_context&>::value, void>::type*) (in /home/arno/Programming/test/a.out)
==266706==    by 0x10DA11: main (in /home/arno/Programming/test/a.out)

Answer 1

https://linux.die.net/man/3/pthread_cond_signal

无论线程当前是否拥有调用 pthread_cond_wait() 或 pthread_cond_timedwait() 的线程在等待期间与条件变量关联的互斥锁，线程都可以调用 pthread_cond_broadcast() 或 pthread_cond_signal() 函数；但是，如果需要可预测的调度行为，则该互斥锁应由调用 pthread_cond_broadcast() 或 pthread_cond_signal() 的线程锁定。

这只是一个诊断警告，指出了围绕条件变量进行低效调度的常见原因。如果您（作为生产者）在发出信号时不保持互斥锁锁定，您会得到虚假唤醒。例如。可能在您解锁互斥锁后立即处理了您对 CV 变量的更新，因此您的信号随后会导致另一个不必要的唤醒，而没有任何更改状态。

为 CV 变量的整个更新以及相反的信号保持锁定互斥锁可以有效地处理，因为消费者已经在互斥锁和 CV 上成对注册，并且直接向 CV 发送信号会使消费者等待 -列出互斥锁，甚至一次都不激活线程。

这只是效率低下，而不是逻辑错误。 Helgrind 仅将其报告为“可疑”，而不是错误。