On Fri, May 8, 2009 at 1:28 AM, seshikanth varma <seshikanthvarma@xxxxxxxxx> wrote:
This is the way i have defined the variables: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");Though every thread has its own copy of registers,the registers eax, ebx ecx and edx are unique by name. So, the threads should be writing to these registers which means that these variables are shared.
I am still confused, as every thread will be having its own register set and this set will get stored in its 'tast_struct' at each context switch and at next run registers will get populated from corresponding 'task_struct'.
So how these variable will get shared between different threads? I am missing any basic thing?
Thanks,
Chetan Nanda
So how these variable will get shared between different threads? I am missing any basic thing?
Thanks,
Chetan Nanda
Regards,SeshikanthOn Fri, May 8, 2009 at 1:21 AM, Daniel Baluta <daniel.baluta@xxxxxxxxx> wrote:
> 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");
Are you sure that this variables are shared among threads?
Please keep in mind that every thread has its own copy of registers.
> 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
>
>
