packaged_task 挂在 operator() 上
packaged_task hanging on operator()
在 Ubuntu 上使用 gcc 4.7.2 编译，使用 -std=c++11 -O0 -pthread 编译，我以某种方式在代码中创建了一个死锁，它似乎永远不会遇到该问题。我有一个线程，它只获取一个锁，然后运行 vector<function<void()>>，调用所有内容。同时，主线程将 std::packaged_task<int()> 一个接一个地推到它上面，并在该任务 future returns 时阻塞。任务本身是微不足道的（打印和返回）。
这是完整的代码。运行应用程序有时会成功，但在几次尝试后会挂起：
#include <iostream>
#include <future>
#include <thread>
#include <vector>
#include <functional>
std::unique_lock<std::mutex> lock() {
static std::mutex mtx;
return std::unique_lock<std::mutex>{mtx};
}
int main(int argc, char** argv)
{
std::vector<std::function<void()>> messages;
std::atomic<bool> running{true};
std::thread thread = std::thread([&]{
while (running) {
auto lk = lock();
std::cout << "[T] locked with " << messages.size() << " messages." << std::endl;
for (auto& fn: messages) {
fn();
}
messages.clear();
}
});
for (int i = 0; i < 1000000; ++i) {
std::packaged_task<int()> task([=]{
std::cout << "[T] returning " << i << std::endl;
return i;
});
{
auto lk = lock();
messages.emplace_back(std::ref(task));
}
task.get_future().get();
}
running = false;
thread.join();
}
示例输出：
[T] returning 127189
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 1 messages.
[T] returning 127190
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 1 messages.
[T] returning 127191
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 1 messages.
... hangs forever ...
这是怎么回事？为什么对 packaged_task::operator() 的调用挂起？僵局在哪里？这是 gcc 错误吗？
[更新] 死锁时，两个线程位于：
线程 1（第 39 行是 task.get_future().get() 行）：
#0 pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:162
#1 0x00007feb01fe800c in __gthread_cond_wait (this=Unhandled dwarf expression opcode 0xf3
)
at [snip]/libstdc++-v3/include/x86_64-unknown-linux-gnu/bits/gthr-default.h:879
#2 std::condition_variable::wait (this=Unhandled dwarf expression opcode 0xf3
) at [snip]/gcc-4.7.2/libstdc++-v3/src/c++11/condition_variable.cc:52
#3 0x0000000000404aff in void std::condition_variable::wait<std::__future_base::_State_base::wait()::{lambda()#1}>(std::unique_lock<std::mutex>&, std::__future_base::_State_base::wait()::{lambda()#1}) (this=0x6111e0, __lock=..., __p=...)
at [snip]gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/condition_variable:93
#4 0x0000000000404442 in std::__future_base::_State_base::wait (this=0x6111a8)
at [snip]gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:331
#5 0x00000000004060fb in std::__basic_future<int>::_M_get_result (this=0x7fffc451daa0)
at [snip]gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:601
#6 0x0000000000405488 in std::future<int>::get (this=0x7fffc451daa0)
at [snip]gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:680
#7 0x00000000004024dc in main (argc=1, argv=0x7fffc451dbb8) at test.cxx:39
和线程 2（第 22 行是 fn() 行）：
#0 pthread_once () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_once.S:95
#1 0x00000000004020f6 in __gthread_once (__once=0x611214, __func=0x401e68 <__once_proxy@plt>)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/x86_64-unknown-linux-gnu/bits/gthr-default.h:718
#2 0x0000000000404db1 in void std::call_once<void (std::__future_base::_State_base::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()()>&, bool&), std::__future_base::_State_base* const, std::reference_wrapper<std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()()> >, std::reference_wrapper<bool> >(std::once_flag&, void (std::__future_base::_State_base::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()()>&, bool&), std::__future_base::_State_base* const&&, std::reference_wrapper<std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()()> >&&, std::reference_wrapper<bool>&&) (__once=..., __f=@0x7feb014fdc10)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/mutex:819
#3 0x0000000000404517 in std::__future_base::_State_base::_M_set_result(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()()>, bool) (this=0x6111a8, __res=..., __ignore_failure=false)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:362
#4 0x0000000000407af0 in std::__future_base::_Task_state<int ()()>::_M_run() (this=0x6111a8)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:1271
#5 0x00000000004076cc in std::packaged_task<int ()()>::operator()() (this=0x7fffc451da30)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:1379
#6 0x000000000040745a in std::_Function_handler<void ()(), std::reference_wrapper<std::packaged_task<int ()()> > >::_M_invoke(std::_Any_data const&) (
__functor=...) at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/functional:1956
#7 0x00000000004051f2 in std::function<void ()()>::operator()() const (this=0x611290)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/functional:2311
#8 0x000000000040232f in operator() (__closure=0x611040) at test.cxx:22
#9 0x0000000000403d8e in _M_invoke<> (this=0x611040)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/functional:1598
#10 0x0000000000403cdb in operator() (this=0x611040)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/functional:1586
#11 0x0000000000403c74 in _M_run (this=0x611028) at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/thread:115
#12 0x00007feb01feae10 in execute_native_thread_routine (__p=Unhandled dwarf expression opcode 0xf3
) at [snip]/gcc-4.7.2/libstdc++-v3/src/c++11/thread.cc:73
#13 0x00007feb018879ca in start_thread (arg=<value optimized out>) at pthread_create.c:300
#14 0x00007feb015e569d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#15 0x0000000000000000 in ?? ()
我无法解释 为什么 你的代码被破坏了，但我确实找到了修复它的方法（存储任务，而不是 std::functions 从任务构造）：
#include <iostream>
#include <future>
#include <thread>
#include <vector>
#include <functional>
#include <unistd.h>
int main(int argc, char** argv)
{
// Let's face it - your lock() function was kinda weird.
std::mutex mtx;
// I've changed this to a vector of tasks, from a vector
// of functions. Seems to have done the job. Not sure exactly
// why but this seems to be the proper way to go.
std::vector<std::packaged_task<int()>> messages;
std::atomic<bool> running{true};
std::thread thread([&]{
while (running) {
std::unique_lock<std::mutex> l{mtx};
std::cout << "[T] locked with " << messages.size() << " messages." << std::endl;
for (auto& fn: messages) {
fn();
}
messages.clear();
}
});
for (int i = 0; i < 1000000; ++i) {
std::packaged_task<int()> task([i]{
std::cout << "[T] returning " << i << std::endl;
return i;
});
// Without grabbing this now, if the thread executed fn()
// before I do f.get() below, it complained about having
// no shared state.
std::future<int> f = task.get_future();
{
std::unique_lock<std::mutex> l{mtx};
messages.emplace_back(std::move(task));
}
f.get();
}
running = false;
thread.join();
}
至少，如果这段代码也死锁了，那我还没有。
看来问题是你在工作线程中销毁了可能在operator() return之前的packaged_task。这很可能是未定义的行为。如果我在等待未来 return 结果后重新获取循环中的互斥锁，该程序对我来说工作正常。这序列化 operator() 和 packaged_task.
的析构函数
在 Ubuntu 上使用 gcc 4.7.2 编译，使用 -std=c++11 -O0 -pthread 编译，我以某种方式在代码中创建了一个死锁，它似乎永远不会遇到该问题。我有一个线程，它只获取一个锁，然后运行 vector<function<void()>>，调用所有内容。同时，主线程将 std::packaged_task<int()> 一个接一个地推到它上面，并在该任务 future returns 时阻塞。任务本身是微不足道的（打印和返回）。
这是完整的代码。运行应用程序有时会成功，但在几次尝试后会挂起：
#include <iostream>
#include <future>
#include <thread>
#include <vector>
#include <functional>
std::unique_lock<std::mutex> lock() {
static std::mutex mtx;
return std::unique_lock<std::mutex>{mtx};
}
int main(int argc, char** argv)
{
std::vector<std::function<void()>> messages;
std::atomic<bool> running{true};
std::thread thread = std::thread([&]{
while (running) {
auto lk = lock();
std::cout << "[T] locked with " << messages.size() << " messages." << std::endl;
for (auto& fn: messages) {
fn();
}
messages.clear();
}
});
for (int i = 0; i < 1000000; ++i) {
std::packaged_task<int()> task([=]{
std::cout << "[T] returning " << i << std::endl;
return i;
});
{
auto lk = lock();
messages.emplace_back(std::ref(task));
}
task.get_future().get();
}
running = false;
thread.join();
}
示例输出：
[T] returning 127189
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 1 messages.
[T] returning 127190
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 1 messages.
[T] returning 127191
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 0 messages.
[T] locked with 1 messages.
... hangs forever ...
这是怎么回事？为什么对 packaged_task::operator() 的调用挂起？僵局在哪里？这是 gcc 错误吗？
[更新] 死锁时，两个线程位于：
线程 1（第 39 行是 task.get_future().get() 行）：
#0 pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:162
#1 0x00007feb01fe800c in __gthread_cond_wait (this=Unhandled dwarf expression opcode 0xf3
)
at [snip]/libstdc++-v3/include/x86_64-unknown-linux-gnu/bits/gthr-default.h:879
#2 std::condition_variable::wait (this=Unhandled dwarf expression opcode 0xf3
) at [snip]/gcc-4.7.2/libstdc++-v3/src/c++11/condition_variable.cc:52
#3 0x0000000000404aff in void std::condition_variable::wait<std::__future_base::_State_base::wait()::{lambda()#1}>(std::unique_lock<std::mutex>&, std::__future_base::_State_base::wait()::{lambda()#1}) (this=0x6111e0, __lock=..., __p=...)
at [snip]gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/condition_variable:93
#4 0x0000000000404442 in std::__future_base::_State_base::wait (this=0x6111a8)
at [snip]gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:331
#5 0x00000000004060fb in std::__basic_future<int>::_M_get_result (this=0x7fffc451daa0)
at [snip]gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:601
#6 0x0000000000405488 in std::future<int>::get (this=0x7fffc451daa0)
at [snip]gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:680
#7 0x00000000004024dc in main (argc=1, argv=0x7fffc451dbb8) at test.cxx:39
和线程 2（第 22 行是 fn() 行）：
#0 pthread_once () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_once.S:95
#1 0x00000000004020f6 in __gthread_once (__once=0x611214, __func=0x401e68 <__once_proxy@plt>)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/x86_64-unknown-linux-gnu/bits/gthr-default.h:718
#2 0x0000000000404db1 in void std::call_once<void (std::__future_base::_State_base::*)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()()>&, bool&), std::__future_base::_State_base* const, std::reference_wrapper<std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()()> >, std::reference_wrapper<bool> >(std::once_flag&, void (std::__future_base::_State_base::*&&)(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()()>&, bool&), std::__future_base::_State_base* const&&, std::reference_wrapper<std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()()> >&&, std::reference_wrapper<bool>&&) (__once=..., __f=@0x7feb014fdc10)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/mutex:819
#3 0x0000000000404517 in std::__future_base::_State_base::_M_set_result(std::function<std::unique_ptr<std::__future_base::_Result_base, std::__future_base::_Result_base::_Deleter> ()()>, bool) (this=0x6111a8, __res=..., __ignore_failure=false)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:362
#4 0x0000000000407af0 in std::__future_base::_Task_state<int ()()>::_M_run() (this=0x6111a8)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:1271
#5 0x00000000004076cc in std::packaged_task<int ()()>::operator()() (this=0x7fffc451da30)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/future:1379
#6 0x000000000040745a in std::_Function_handler<void ()(), std::reference_wrapper<std::packaged_task<int ()()> > >::_M_invoke(std::_Any_data const&) (
__functor=...) at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/functional:1956
#7 0x00000000004051f2 in std::function<void ()()>::operator()() const (this=0x611290)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/functional:2311
#8 0x000000000040232f in operator() (__closure=0x611040) at test.cxx:22
#9 0x0000000000403d8e in _M_invoke<> (this=0x611040)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/functional:1598
#10 0x0000000000403cdb in operator() (this=0x611040)
at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/functional:1586
#11 0x0000000000403c74 in _M_run (this=0x611028) at [snip]/gcc-4.7.2/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/thread:115
#12 0x00007feb01feae10 in execute_native_thread_routine (__p=Unhandled dwarf expression opcode 0xf3
) at [snip]/gcc-4.7.2/libstdc++-v3/src/c++11/thread.cc:73
#13 0x00007feb018879ca in start_thread (arg=<value optimized out>) at pthread_create.c:300
#14 0x00007feb015e569d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#15 0x0000000000000000 in ?? ()
我无法解释 为什么 你的代码被破坏了，但我确实找到了修复它的方法（存储任务，而不是 std::functions 从任务构造）：
#include <iostream>
#include <future>
#include <thread>
#include <vector>
#include <functional>
#include <unistd.h>
int main(int argc, char** argv)
{
// Let's face it - your lock() function was kinda weird.
std::mutex mtx;
// I've changed this to a vector of tasks, from a vector
// of functions. Seems to have done the job. Not sure exactly
// why but this seems to be the proper way to go.
std::vector<std::packaged_task<int()>> messages;
std::atomic<bool> running{true};
std::thread thread([&]{
while (running) {
std::unique_lock<std::mutex> l{mtx};
std::cout << "[T] locked with " << messages.size() << " messages." << std::endl;
for (auto& fn: messages) {
fn();
}
messages.clear();
}
});
for (int i = 0; i < 1000000; ++i) {
std::packaged_task<int()> task([i]{
std::cout << "[T] returning " << i << std::endl;
return i;
});
// Without grabbing this now, if the thread executed fn()
// before I do f.get() below, it complained about having
// no shared state.
std::future<int> f = task.get_future();
{
std::unique_lock<std::mutex> l{mtx};
messages.emplace_back(std::move(task));
}
f.get();
}
running = false;
thread.join();
}
至少，如果这段代码也死锁了，那我还没有。
看来问题是你在工作线程中销毁了可能在operator() return之前的packaged_task。这很可能是未定义的行为。如果我在等待未来 return 结果后重新获取循环中的互斥锁，该程序对我来说工作正常。这序列化 operator() 和 packaged_task.