为什么GDB中“futex断点”和write()会互相影响

Question

当我使用 gdb 调试带有输出的 futex 锁时，程序陷入了一个奇怪的循环。

#include <mutex>
#include <iostream>
#include <thread>
#include <unistd.h>



volatile int counter(0); // non-atomic counter

std::mutex mtx;

void increases10k()
{
        for(int i=0;i<100000000;i++)
        {

                sleep(1);
                std::cout << "The ID of this thread is: " << std::this_thread::get_id() << std::endl;
                std::cout  << counter <<std::endl;
                mux.lock();

                ++counter;
                std::cout  << counter <<std::endl;
             
                mtx.unlock();

        }

}

int main(int argc,char **argv)
{
        std::thread threads[10];
        for(int i=0;i<10;i++)
        {
                threads[i]=std::thread(increases10k);
        }
        for(auto& th:threads)
                th.join();
        std::cout << " successful increases of the counter " << counter <<std::endl;
        return 0;

}

我使用gdb命令

catch system futex

来检查系统调用futex。

然后我使用

c

来保持程序运行，但是程序停止了输出并陷入了奇怪的调用和ret循环中。

Thread 8 "a.out" hit Catchpoint 1 (call to syscall futex), __lll_lock_wait_private ()
    at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
95      in ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S
[Switching to Thread 0x7f0db5ffb700 (LWP 8923)]

Thread 9 "a.out" hit Catchpoint 1 (call to syscall futex), __lll_lock_wait_private ()
    at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
95      in ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S
[Switching to Thread 0x7f0db7fff700 (LWP 8919)]

Thread 5 "a.out" hit Catchpoint 1 (returned from syscall futex), __lll_lock_wait_private ()
    at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
95      in ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S
[Switching to Thread 0x7f0dbd044700 (LWP 8917)]

Thread 3 "a.out" hit Catchpoint 1 (call to syscall futex), __lll_lock_wait_private ()
    at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
95      in ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S
[Switching to Thread 0x7f0dbd845700 (LWP 8916)]

Thread 2 "a.out" hit Catchpoint 1 (returned from syscall futex), __lll_lock_wait_private ()
    at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
95      in ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S
[Switching to Thread 0x7f0dbd044700 (LWP 8917)]

Thread 3 "a.out" hit Catchpoint 1 (returned from syscall futex), __lll_lock_wait_private ()
    at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
95      in ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S

无穷无尽。我还注意到一个线程被困在 write() 中，但按 c 无法让它继续。

Id   Target Id         Frame
  1    Thread 0x7f0dbe8ac5c0 (LWP 8915) "a.out" 0x00007f0dbe48a6dd in pthread_join (threadid=139696990869248, thread_return=0x0)
    at pthread_join.c:90
  2    Thread 0x7f0dbd845700 (LWP 8916) "a.out" __lll_lock_wait_private () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
  3    Thread 0x7f0dbd044700 (LWP 8917) "a.out" __lll_lock_wait_private () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
  4    Thread 0x7f0dbc843700 (LWP 8918) "a.out" __lll_lock_wait_private () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
  5    Thread 0x7f0db7fff700 (LWP 8919) "a.out" __lll_lock_wait_private () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
  6    Thread 0x7f0db77fe700 (LWP 8920) "a.out" __lll_lock_wait_private () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
  7    Thread 0x7f0db6ffd700 (LWP 8921) "a.out" __lll_lock_wait_private () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
  8    Thread 0x7f0db67fc700 (LWP 8922) "a.out" __lll_lock_wait_private () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
  9    Thread 0x7f0db5ffb700 (LWP 8923) "a.out" __lll_lock_wait_private () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95
* 10   Thread 0x7f0db57fa700 (LWP 8924) "a.out" 0x00007f0dbd92198d in write () at ../sysdeps/unix/syscall-template.S:84
  11   Thread 0x7f0db4ff9700 (LWP 8925) "a.out" __lll_lock_wait_private () at ../sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:95

当我注释掉输出时，程序在 gdb 中使用

c

可以正常工作。

我不明白为什么“futex 处的断点”和 write() 会互相影响。

Answer 1

当我使用gdb调试futex锁时

请注意，使用 GDB 调试 futex 锁（或任何其他多线程问题）是不可能的——您需要通过构造来使程序正确。

您可以做的唯一调试就是了解程序死锁后的位置。

无穷无尽

你期待什么？每次调用

mux.lock()

和

mux.unlock()

may 都会执行

futex

调用，并且您正在执行 2 * 10 * 100'000'000 个调用（还有一个额外的因子 2，因为 GDB 在 entry 停止）并从系统调用退出）。

当然你会永远被困在那里。

http://xyproblem.info在这里似乎很合适。