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
#include
#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(" ==> | %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 in advance...