OSDev.org

[root@dev]# cat race.found.log
RACE!: ['75330', '2', '75877']
RACE!: ['103486', '2', '104581']
RACE!: ['34712', '3', '35232']
RACE!: ['105220', '5', '105751']

[root@dev]# egrep -ir "75330:2" file.all.log -A 2 -B 2
threadNo. 0:75321:4:75325
threadNo. 0:75325:5:75330
threadNo. 0:75330:2:75877
threadNo. 0:75877:2:75879
threadNo. 0:75879:4:75883

#include <list>
#include <mutex>
#include <algorithm>
#include <thread>
#include <iostream>
#include <fstream>
#include <sstream>
#include <unistd.h>

using namespace std;

std::mutex mutex_sum;
int sum = 0;

void hello(int pId, int pStat[])
{
    int pNum;
    int i;

    std::cout << pId << ": Hello CONCURRENT WORLD " << endl;

    for (int i = 0; i < 1000000; i++ ) {
        pNum  = rand() % 4 + 2;

        cout << pId << ": " << ", loop: " << i << endl;
        // read + sum + read and write to file.

        ofstream myfile;
        ostringstream oss;
        oss << "file-" << pId << "-" << i;
        myfile.open(oss.str());
        myfile << "threadNo. " << pId << ":" << sum;
        sum += pNum;
        myfile << ":" << pNum << ":" << sum << endl;
        pStat[pId]  = 1;
        myfile.close();
    }
    std::cout << pId << ": Done sleeping exiting now..." << endl;
}

int main()
{
    // Declare, initialize variables.

    int i;
    const int CONFIG_THREAD_COUNT = 2;
    int stat[CONFIG_THREAD_COUNT];
    int sum = 0;

    // launch threads.

    for ( i = 0; i < CONFIG_THREAD_COUNT; i ++ ) {
        stat[i] = 0;
        std::thread t(hello, i, stat);
        t.detach();
    }
    cout << "Checking thread status-s..." << endl;

     while (sum != CONFIG_THREAD_COUNT)  {
        sum = 0;

        for (i = 0; i < CONFIG_THREAD_COUNT; i++) {
            cout << stat[i] << ", ";
            sum += stat[i];
        }

        cout << "main(): sum: " << sum << ". waiting for all threads to finish..." << endl;
        usleep(2 * 1000000);
    }

    return 0;
}

fp = open("file.all.log")
fpOut = open("race.found.log", 'w')

if not fp:
    print "Failed to open."

line = fp.readline().strip()

counter = 0

while line:
    print line
    operands= line.split(":")[1:4]
    print "operands: ", operands

    if int(operands[0]) + int(operands[1]) != int(operands[2]):
        fpOut.write("RACE!: " + str(operands) + "\n")
        counter += 1

    line = fp.readline().strip()

if counter == 0:
    fpOut.write("No race condition found.")
    print "No race condition found."
else:
    print "At least one race condition found: ", counter

fp.close()
fpOut.close()

nullplan wrote:Nothing in your code actually locks the mutex. You need to take the lock after opening myfile at the latest, and release it after the last access to sum at the earliest.

What is the point of this exercise?

ggodw000 wrote:you have a wandering eye?

ggodw000 wrote:you have a wandering eye?

nullplan wrote:

ggodw000 wrote:you have a wandering eye?

I am unfamiliar with that idiom. What did you mean to say?

Schol-R-LEA wrote:I know the idiom as it is used in English, but in this context it makes no sense ; it usually refers to romantic infidelity (i.e., someone who keeps looking at attractive individuals other than their current partner, with the implication that they are doing more than just looking). Perhaps ggodw000 thinks that nullplan is going to cheat on their significant other with a hot piece of software?

nullplan wrote:NP: Die Ärzte - Alles für dich (look it up!)

Schol-R-LEA wrote:TBH, that question (why you would deliberately create a race condition, something which mutual exclusion primitives are intended to help avoid by allowing only one process access at a time) was the first thing that came to mind for me, too. I'd ignored the thread up until now because the title didn't even make sense to me.

I assume that there is some reason why you would intentionally set up a race condition, but I am at a loss as to why. The only thing I can think of is that you are trying to learn how to detect them, but even that makes no sense - unlike, say, a deadlock condition, it is always possible to prevent a race condition through any of the mutual exclusion approaches in common use. Race conditions come about because of failure to use them when needed, or using them incorrectly, not because of an inherent limitation in mutual exclusion.

I suppose it could be just to see what a race condition's effects are, but that doesn't make much sense to me - the well known examples should be sufficient to demonstrate that.

So yeah, I'm just as confused as nullplan on this.

Your post does seem to allude to the possibility that this was for testing the detection code, which you were using to debug a program you know has a race condition which you couldn't pin down. However, you never stated this outright in the post, and the title implies that the race condition itself is the goal, which would seem pretty bizarre.

The confusion in this thread seems to involve an XY problem. Please, ggodw000, could you tell us what you are actually trying to accomplish?

nullplan wrote:

ggodw000 wrote:you have a wandering eye?

I am unfamiliar with that idiom. What did you mean to say?

I know the idiom as it is used in English, but in this context it makes no sense ; it usually refers to romantic infidelity (i.e., someone who keeps looking at attractive individuals other than their current partner, with the implication that they are doing more than just looking). Perhaps ggodw000 thinks that nullplan is going to cheat on their significant other with a hot piece of software?

ggodw000 wrote: [...] now it is adding lot of numbers through each loop, the end result will eventually be the same even though it is a race condition, it is a benign race condition if one is only interested in the final result not the intermediate. [...]

V := 0 // shared variable

Thread code:
  tmp := V + 1
  V <- tmp

Thread A:
  tmp := V + 1 = 0 + 1 = 1
  [Thread A is suspended]

Thread B:
  tmp := V + 1 = 0 + 1 = 1 // V has not been updated yet
  V <- 1
  [Thread B is suspended]

Thread A:
  V <- tmp // tmp is still 1, the result is not correct

Schol-R-LEA wrote:I'm still not sure why you want to create a race condition deliberately, though. I don't see what it teaches you, since the whole point in studying them in concurrent programming is to learn how to prevent them.

Race conditions are easy enough to create by accident, and I'm not sure if an exercise in creating them on purpose would give you any real knowledge that isn't inherent simply in knowing that race conditions exist at all. Since different circumstances lead to very different outcomes, there is also no general approach to mitigating or eliminating them which can be found from such a study, either.

So again: what are you trying to accomplish with this?