Question

为什么根据valgrind有以下代码存在竞争条件？

#include <iostream>
#include <ctime>
#include <tr1/random>
#include <omp.h>

using namespace std;

tr1::mt19937 randgen;

int random(int min, int max)
{
    int number;
    if (min!=max)
    {
        #pragma omp critical(randgen)
        number = ((randgen() % (max - min)) + min);
        return number;

    } else
        return min;
}

int main(int argc, char *argv[])
{
    omp_set_num_threads(4);
    randgen.seed(time(NULL));
    #pragma omp parallel for
    for (int i = 0; i < 10; i++)
    {
        random(10,100);
    }

    return 0;
}

编译为：

g++ -O3 -g -Wall -c -fmessage-length=0 -fopenmp -MMD -MP -MF"main.d" -MT"main.d" -o "main.o" "../main.cpp"
g++  -o "test"  ./main.o   -lgomp

Valgrind结果

valgrind --tool=drd --check-stack-var=yes --read-var-info=yes ./test
==19561== drd, a thread error detector
==19561== Copyright (C) 2006-2010, and GNU GPL'd, by Bart Van Assche.
==19561== Using Valgrind-3.6.1-Debian and LibVEX; rerun with -h for copyright info
==19561== Command: ./test
==19561== 
==19561== Thread 3:
==19561== Conflicting load by thread 3 at 0x00603420 size 4
==19561==    at 0x400A10:     _ZNSt3tr116mersenne_twisterImLi32ELi624ELi397ELi31ELm2567483615ELi11ELi7ELm2636928640ELi15E    Lm4022730752ELi18EEclEv.constprop.2 (random.tcc:323)
==19561==    by 0x400BB9: main._omp_fn.0 (main.cpp:16)
==19561==    by 0x4E3FEC9: ??? (in /usr/lib/x86_64-linux-gnu/libgomp.so.1.0.0)
==19561==    by 0x4C2A803: vgDrd_thread_wrapper (drd_pthread_intercepts.c:281)
==19561==    by 0x58FDEFB: start_thread (pthread_create.c:304)
==19561==    by 0x543059C: clone (clone.S:112)
==19561== Location 0x603420 is 0 bytes inside randgen._M_p,
==19561== a global variable declared at main.cpp:8
==19561== Other segment start (thread 1)
==19561==    at 0x4C2AE7D: pthread_create@* (drd_pthread_intercepts.c:440)
==19561==    by 0x4E402FB: ??? (in /usr/lib/x86_64-linux-gnu/libgomp.so.1.0.0)
==19561==    by 0x400898: main (main.cpp:27)
==19561== Other segment end (thread 1)
==19561==    at 0x4E41550: ??? (in /usr/lib/x86_64-linux-gnu/libgomp.so.1.0.0)
==19561==    by 0x4E406CD: ??? (in /usr/lib/x86_64-linux-gnu/libgomp.so.1.0.0)
==19561==    by 0x4008A4: main (main.cpp:27)

我认为通过使用#pragma omp critical一次只有1个线程可以调用给定的函数？我很困惑。

Answer 1

您可以在Valgrind manual：

中找到答案

DRD仅支持已配置的libgomp库这个选项[--disable-linux-futex]和符号信息当下。对于大多数Linux发行版而言，这意味着您必须这样做重新编译GCC。

tr1 :: randgen（）的竞争条件，即使#pragma omp critical也是如此

1 个答案: