Not able to generate race condition... please help

[seshikanth varma <seshikanthvarma@xxxxxxxxx>]

Hi,

I am learning linux kernel. I have written a simple program to understand the usage of mutex variable. 

Ideally the following program should generate race condition and should produce the different values on i,j,k and l rather than i = j = k = l = 100(LOOPCONSTANT * Number of threads) on usage of mutex. Here shared variables between threads are i,j,k and l.

My kernel is not SMP.  Can u please tell me where am i going wrong?

======================================================================================================

#include <pthread.h>
#include <stdio.h>
#define checkResults(string, val) {             \
    if (val) {                                     \
        printf("Failed with %d at %s", val, string); \
        exit(1);                                     \
    }                                              \
}
//#define            LOOPCONSTANT     100000
#define            LOOPCONSTANT     10
#define            THREADS          10

pthread_mutex_t    mutex = PTHREAD_MUTEX_INITIALIZER;
volatile int                i asm("eax"); /* For using eax ebx ecx & edx for i, j , k  and l respectively.

                                           *  All the threads shud use eax for i, ebx for j ...
                                           */ 
volatile int                j asm("ebx");
volatile int                k asm("ecx");
volatile int                l asm("edx");
int                uselock=0;

void *threadfunc(void *parm)
{
    int   loop = 0;
    int   rc;

    for (loop=0; loop<LOOPCONSTANT; ++loop) {
        if (uselock) {
            rc = pthread_mutex_lock(&mutex);
            checkResults("pthread_mutex_lock()\n", rc);
        }
        ++i; ++j; ++k; ++l;
        if (uselock) {
            rc = pthread_mutex_unlock(&mutex);
            checkResults("pthread_mutex_unlock()\n", rc);
        }
    }
    return NULL;
}

int main(int argc, char **argv)
{
    pthread_t             threadid[THREADS];
    int                   rc=0;

    int                   loop=0;
    pthread_attr_t        pta;

    printf("Entering testcase\n");
    printf("Give any number of parameters to show data corruption\n");
    if (argc != 1) {
        printf("A parameter was specified, no serialization is being done!\n");
        uselock = 0;
    }

    pthread_attr_init(&pta);
    pthread_attr_setdetachstate(&pta, PTHREAD_CREATE_JOINABLE);

    printf("Creating %d threads\n", THREADS);
    for (loop=0; loop<THREADS; ++loop) {
        rc = pthread_create(&threadid[loop], &pta, threadfunc, NULL);
        checkResults("pthread_create()\n", rc);
    }

    printf("Wait for results\n");
    for (loop=0; loop<THREADS; ++loop) {
        rc = pthread_join(threadid[loop], NULL);
        checkResults("pthread_join()\n", rc);
    } 

    printf("Cleanup and show results\n");
    pthread_attr_destroy(&pta);
    pthread_mutex_destroy(&mutex);

    printf("\nUsing %d threads and LOOPCONSTANT = %d\n",
           THREADS, LOOPCONSTANT);
    printf("Values are: (should be %d)\n", THREADS * LOOPCONSTANT);
    printf("  ==> seshikanth | %d, %d, %d, %d\n", i, j, k, l);

    printf("Main completed\n");
    return 0;

} 

======================================================================================================

O/p of the above program:

---------------------------------------

with userlock = 0 (i.e., shud hit race condition)

--------------------------------------------------------------------

gcc check.c -pthread

bash-2.05b$ ./a.out

Creating 10 threads

Wait for results

Cleanup and show results

Using 10 threads and LOOPCONSTANT = 10

Values are: (should be 100)

  ==> | 100, 100, 100, 100 <<<<<<<<<<<==========*******

Main completed

bash-2.05b$

=====================================

with userlock = 1;

-------------------------

I am getting the same o/p which is correct....

Any help regarding this is greatly appreciated.

Thanks,

Seshikanth