Race condition on tr1::randgen() even with #pragma omp critical - c++

Question

Why there is a race condition according to valgrind with the following code?
#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;
}
Compiled as:
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 results
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)
I thought that by using #pragma omp critical only 1 thread at a time can call the given function? I'm confused.

Answer 1

You can find the answer in the Valgrind manual:
DRD only supports libgomp libraries that have been configured with
this option [--disable-linux-futex] and in which symbol information is
present. For most Linux distributions this means that you will have to
recompile GCC.