std::async 导致死锁？

Question

std::async 导致死锁？

我试图在繁重的工作负载应用程序中使用 std::async 来提高性能，但我不时遇到死锁。我调试了很长时间，我几乎可以肯定我的代码很好，而且 std 库似乎有问题。

于是我写了一个简单的测试程序来作证：

#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
#include <future>
#include <string>
#include <mutex>
#include <unistd.h>
#include <atomic>
#include <iomanip>

std::atomic_long numbers[6];

void add(std::atomic_long& n)
{
    ++n;
}

void func2(std::atomic_long& n)
{
    for (auto i = 0L; i < 1000000000000L; ++i)
    {
        std::async(std::launch::async, [&] {add(n);});   // Small task, I want to run them simultaneously
    }
}

int main()
{
    std::vector<std::future<void>> results;
    for (int i = 0; i < 6; ++i)
    {
        auto& n = numbers[i];
        results.push_back(std::async(std::launch::async, [&n] {func2(n);}));
    }

    while (true)
    {
        sleep(1);
        for (int i = 0; i < 6; ++i)
            std::cout << std::setw(20) << numbers[i] << " ";
        std::cout << std::endl;
    }

    for (auto& r : results)
    {
        r.wait();
    }
    return 0;
}

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该程序将产生如下输出：

              763700               779819               754005               763287               767713               748994 
              768822               785172               759678               769393               772956               754469 
              773529               789382               763524               772704               776398               757864 
              778560               794419               768580               777507               781542               762991 
              782056               795578               771704               780554               784865               766162 
              801633               812610               788111               802617               803661               784894

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一段时间（几分钟或几小时）后，如果出现死锁，输出将如下所示：

             4435337              4452421              4507907              4501378              2549550              4462899 
             4441213              4457648              4514424              4506626              2549550              4468019 
             4446301              4462675              4519272              4511889              2549550              4473266 
             4453940              4470304              4526382              4519513              2549550              4480872 
             4461095              4477708              4533272              4526901              2549550              4488313 
             4470974              4488287              4543442              4537286              2549550              4498733

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第五列被冻结。

一天后，变成了这样：

            23934912             23967635             24007250             23931203              2549550           3249788689 
            23934912             23967635             24007250             23931203              2549550           3249816818 
            23934912             23967635             24007250             23931203              2549550           3249835009 
            23934912             23967635             24007250             23931203              2549550           3249860262 
            23934912             23967635             24007250             23931203              2549550           3249894331

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除了最后一列，几乎所有列都冻结了。看起来真的很奇怪。

我在 Linux、macOS、FreeBSD 上运行它，结果是：

macOS:10.15.2, Clang:11.0.0, 无死锁
FreeBSD:12.0, Clang:6.0.1, 死锁
Linux: ubuntu 5.0.0-37, g++:7.4.0, 无死锁
Linux：ubuntu 4.4.0-21，Clang:3.8.0，死锁

在 gdb 中，调用堆栈是：

(gdb) thread apply all bt

Thread 10 (LWP 100467 of process 37763):
#0  0x000000080025c630 in ?? () from /lib/libthr.so.3
#1  0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x7fff4ad57000

Thread 9 (LWP 100464 of process 37763):
#0  0x000000080046fafa in _umtx_op () from /lib/libc.so.7
#1  0x0000000800264912 in ?? () from /lib/libthr.so.3
#2  0x000000080031f9f9 in std::__1::mutex::unlock() () from /usr/lib/libc++.so.1
#3  0x00000008002e8f55 in std::__1::__assoc_sub_state::set_value() () from /usr/lib/libc++.so.1
#4  0x00000000002053e1 in std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::__execute() ()
#5  0x0000000000205763 in void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >*> >(void*) ()
#6  0x000000080025c776 in ?? () from /lib/libthr.so.3
#7  0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x7fff6944a000

Thread 8 (LWP 100431 of process 37763):
#0  0x000000080046fafa in _umtx_op () from /lib/libc.so.7
#1  0x0000000800264912 in ?? () from /lib/libthr.so.3
#2  0x000000080031f9f9 in std::__1::mutex::unlock() () from /usr/lib/libc++.so.1
#3  0x00000008002e8f55 in std::__1::__assoc_sub_state::set_value() () from /usr/lib/libc++.so.1
#4  0x00000000002053e1 in std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::__execute() ()
#5  0x0000000000205763 in void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >*> >(void*) ()
#6  0x000000080025c776 in ?? () from /lib/libthr.so.3
#7  0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x7fffc371a000

Thread 7 (LWP 100657 of process 37763):
#0  0x000000080026a66c in ?? () from /lib/libthr.so.3
#1  0x000000080025e731 in ?? () from /lib/libthr.so.3
#2  0x0000000800268388 in ?? () from /lib/libthr.so.3
#3  0x000000080032de72 in std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) () from /usr/lib/libc++.so.1
#4  0x00000008002e971b in std::__1::__assoc_sub_state::wait() () from /usr/lib/libc++.so.1
#5  0x0000000000205389 in std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::__on_zero_shared() ()
#6  0x000000000020346b in func2(std::__1::atomic<long>&) ()
#7  0x0000000000206f18 in main::$_1::operator()() const ()
#8  0x0000000000206eed in void std::__1::__async_func<main::$_1>::__execute<>(std::__1::__tuple_indices<>) ()
#9  0x0000000000206ea5 in std::__1::__async_func<main::$_1>::operator()() ()
#10 0x0000000000206df3 in std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::__execute() ()
#11 0x0000000000207183 in void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >*> >(void*) ()
#12 0x000000080025c776 in ?? () from /lib/libthr.so.3
#13 0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x7fffdf5f9000

Thread 6 (LWP 100656 of process 37763):
#0  0x000000080026a66c in ?? () from /lib/libthr.so.3
#1  0x000000080025e731 in ?? () from /lib/libthr.so.3
#2  0x0000000800268388 in ?? () from /lib/libthr.so.3
#3  0x000000080032de72 in std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) () from /usr/lib/libc++.so.1
#4  0x00000008002e971b in std::__1::__assoc_sub_state::wait() () from /usr/lib/libc++.so.1
#5  0x0000000000205389 in std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::__on_zero_shared() ()
#6  0x0000000000207a22 in std::__1::__release_shared_count::operator()(std::__1::__shared_count*) ()
#7  0x00000000002044f4 in std::__1::future<void> std::__1::__make_async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >(std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0>&&) ()
#8  0x00000000002035ea in std::__1::future<std::__1::__invoke_of<std::__1::decay<func2(std::__1::atomic<long>&)::$_0>::type>::type> std::__1::async<func2(std::__1::atomic<long>&)::$_0>(std::__1::launch, func2(std::__1::atomic<long>&)::$_0&&) ()
#9  0x0000000000203462 in func2(std::__1::atomic<long>&) ()
#10 0x0000000000206f18 in main::$_1::operator()() const ()
#11 0x0000000000206eed in void std::__1::__async_func<main::$_1>::__execute<>(std::__1::__tuple_indices<>) ()
#12 0x0000000000206ea5 in std::__1::__async_func<main::$_1>::operator()() ()
#13 0x0000000000206df3 in std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::__execute() ()
#14 0x0000000000207183 in void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >*> >(void*) ()
#15 0x000000080025c776 in ?? () from /lib/libthr.so.3
#16 0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x7fffdf7fa000

Thread 5 (LWP 100655 of process 37763):
#0  0x000000080026a66c in ?? () from /lib/libthr.so.3
#1  0x000000080025e731 in ?? () from /lib/libthr.so.3
#2  0x0000000800268388 in ?? () from /lib/libthr.so.3
#3  0x000000080032de72 in std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) () from /usr/lib/libc++.so.1
#4  0x00000008002e971b in std::__1::__assoc_sub_state::wait() () from /usr/lib/libc++.so.1
#5  0x0000000000205389 in std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::__on_zero_shared() ()
#6  0x0000000000207a22 in std::__1::__release_shared_count::operator()(std::__1::__shared_count*) ()
#7  0x00000000002044f4 in std::__1::future<void> std::__1::__make_async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >(std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0>&&) ()
#8  0x00000000002035ea in std::__1::future<std::__1::__invoke_of<std::__1::decay<func2(std::__1::atomic<long>&)::$_0>::type>::type> std::__1::async<func2(std::__1::atomic<long>&)::$_0>(std::__1::launch, func2(std::__1::atomic<long>&)::$_0&&) ()
#9  0x0000000000203462 in func2(std::__1::atomic<long>&) ()
#10 0x0000000000206f18 in main::$_1::operator()() const ()
#11 0x0000000000206eed in void std::__1::__async_func<main::$_1>::__execute<>(std::__1::__tuple_indices<>) ()
#12 0x0000000000206ea5 in std::__1::__async_func<main::$_1>::operator()() ()
#13 0x0000000000206df3 in std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::__execute() ()
#14 0x0000000000207183 in void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >*> >(void*) ()
#15 0x000000080025c776 in ?? () from /lib/libthr.so.3
#16 0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x7fffdf9fb000

Thread 4 (LWP 100654 of process 37763):
#0  0x000000080026a66c in ?? () from /lib/libthr.so.3
#1  0x000000080025e731 in ?? () from /lib/libthr.so.3
#2  0x0000000800268388 in ?? () from /lib/libthr.so.3
#3  0x000000080032de72 in std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) () from /usr/lib/libc++.so.1
#4  0x00000008002e971b in std::__1::__assoc_sub_state::wait() () from /usr/lib/libc++.so.1
#5  0x0000000000205389 in std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::__on_zero_shared() ()
#6  0x0000000000207a22 in std::__1::__release_shared_count::operator()(std::__1::__shared_count*) ()
#7  0x00000000002044f4 in std::__1::future<void> std::__1::__make_async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >(std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0>&&) ()
#8  0x00000000002035ea in std::__1::future<std::__1::__invoke_of<std::__1::decay<func2(std::__1::atomic<long>&)::$_0>::type>::type> std::__1::async<func2(std::__1::atomic<long>&)::$_0>(std::__1::launch, func2(std::__1::atomic<long>&)::$_0&&) ()
#9  0x0000000000203462 in func2(std::__1::atomic<long>&) ()
#10 0x0000000000206f18 in main::$_1::operator()() const ()
#11 0x0000000000206eed in void std::__1::__async_func<main::$_1>::__execute<>(std::__1::__tuple_indices<>) ()
#12 0x0000000000206ea5 in std::__1::__async_func<main::$_1>::operator()() ()
#13 0x0000000000206df3 in std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::__execute() ()
#14 0x0000000000207183 in void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >*> >(void*) ()
#15 0x000000080025c776 in ?? () from /lib/libthr.so.3
#16 0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x7fffdfbfc000

Thread 3 (LWP 100653 of process 37763):
#0  0x000000080026a66c in ?? () from /lib/libthr.so.3
#1  0x000000080025e731 in ?? () from /lib/libthr.so.3
#2  0x0000000800268388 in ?? () from /lib/libthr.so.3
#3  0x000000080032de72 in std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) () from /usr/lib/libc++.so.1
#4  0x00000008002e971b in std::__1::__assoc_sub_state::wait() () from /usr/lib/libc++.so.1
#5  0x0000000000205389 in std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::__on_zero_shared() ()
#6  0x0000000000207a22 in std::__1::__release_shared_count::operator()(std::__1::__shared_count*) ()
#7  0x00000000002044f4 in std::__1::future<void> std::__1::__make_async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >(std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0>&&) ()
#8  0x00000000002035ea in std::__1::future<std::__1::__invoke_of<std::__1::decay<func2(std::__1::atomic<long>&)::$_0>::type>::type> std::__1::async<func2(std::__1::atomic<long>&)::$_0>(std::__1::launch, func2(std::__1::atomic<long>&)::$_0&&) ()
#9  0x0000000000203462 in func2(std::__1::atomic<long>&) ()
#10 0x0000000000206f18 in main::$_1::operator()() const ()
#11 0x0000000000206eed in void std::__1::__async_func<main::$_1>::__execute<>(std::__1::__tuple_indices<>) ()
#12 0x0000000000206ea5 in std::__1::__async_func<main::$_1>::operator()() ()
#13 0x0000000000206df3 in std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::__execute() ()
#14 0x0000000000207183 in void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >*> >(void*) ()
#15 0x000000080025c776 in ?? () from /lib/libthr.so.3
#16 0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x7fffdfdfd000

Thread 2 (LWP 100652 of process 37763):
#0  0x000000080026a66c in ?? () from /lib/libthr.so.3
#1  0x000000080025e731 in ?? () from /lib/libthr.so.3
#2  0x0000000800268388 in ?? () from /lib/libthr.so.3
#3  0x000000080032de72 in std::__1::condition_variable::wait(std::__1::unique_lock<std::__1::mutex>&) () from /usr/lib/libc++.so.1
#4  0x00000008002e971b in std::__1::__assoc_sub_state::wait() () from /usr/lib/libc++.so.1
#5  0x0000000000205389 in std::__1::__async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >::__on_zero_shared() ()
#6  0x0000000000207a22 in std::__1::__release_shared_count::operator()(std::__1::__shared_count*) ()
#7  0x00000000002044f4 in std::__1::future<void> std::__1::__make_async_assoc_state<void, std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0> >(std::__1::__async_func<func2(std::__1::atomic<long>&)::$_0>&&) ()
#8  0x00000000002035ea in std::__1::future<std::__1::__invoke_of<std::__1::decay<func2(std::__1::atomic<long>&)::$_0>::type>::type> std::__1::async<func2(std::__1::atomic<long>&)::$_0>(std::__1::launch, func2(std::__1::atomic<long>&)::$_0&&) ()
#9  0x0000000000203462 in func2(std::__1::atomic<long>&) ()
#10 0x0000000000206f18 in main::$_1::operator()() const ()
#11 0x0000000000206eed in void std::__1::__async_func<main::$_1>::__execute<>(std::__1::__tuple_indices<>) ()
#12 0x0000000000206ea5 in std::__1::__async_func<main::$_1>::operator()() ()
#13 0x0000000000206df3 in std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::__execute() ()
#14 0x0000000000207183 in void* std::__1::__thread_proxy<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >::*)(), std::__1::__async_assoc_state<void, std::__1::__async_func<main::$_1> >*> >(void*) ()
#15 0x000000080025c776 in ?? () from /lib/libthr.so.3
#16 0x0000000000000000 in ?? ()
Backtrace stopped: Cannot access memory at address 0x7fffdfffe000

Thread 1 (LWP 100148 of process 37763):
#0  0x00000008004f984a in _nanosleep () from /lib/libc.so.7
#1  0x000000080025f17c in ?? () from /lib/libthr.so.3
#2  0x000000080045fe0b in sleep () from /lib/libc.so.7
#3  0x0000000000203b7b in main ()

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好像很多线程卡在了std::__1::condition_variable::wait，不合理，在测试代码中，根本没有使用任何条件。

有人能告诉我，我做错了还是标准库中有错误？

谢谢。这个例子没有完全模仿我的程序的实际行为。我把它简化了太多。

现在我添加未来向量，这更像是它：

void func2(std::atomic_long& n)
{
    std::vector<std::future<void>> rs;
    for (auto i = 0L; i < 1000000000000L; ++i)
    {
        rs.push_b

Answer 1

gre*_*f82 1

您没有考虑的返回值std::async，返回的 future 将阻止任何执行，直到您开始的任务结束std::async。该程序按编写方式执行，并没有达到您的预期。此外，您正在使用递归调用 std::async ，并且不允许它生成一个新线程，它可以管理一个池，因此如果池很忙，您的程序显然会冻结，因为您正在执行的循环非常长。如果您想要更多控制，可以将 std::thread 与 std::packaged_task 一起使用

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