On Fri, May 8, 2009 at 1:28 AM, seshikanth varma
<[email protected]>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
>
> Regards,
> Seshikanth
>
>
> On Fri, May 8, 2009 at 1:21 AM, Daniel Baluta <[email protected]>wrote:
>
>> On Thu, May 7, 2009 at 10:38 PM, seshikanth varma
>> <[email protected]> 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
>> >
>> >
>>
>
>
