Sauron@Mordor: Setting up shared memory with robust pthread_mutex and pthread

I've tried to experiment with shared memory.
Eventually, I figured out that you can use pthread_mutex and pthread_contion to synchronize the access of a shared memory.

During the process, I found out that it is very important to make the pthread_mutex "robust".

This so called robust mutex is important because this mutex can be shared by multiple processes, and therefore if one process crashes without releasing the mutex, all the other process could be stuck forever.

A robust mutex will release it's possession to the next process, if the current mutex possessing process crashes.
The pthread_mutex_lock() of the new mutex possessing process, will return with "EOWNERDEAD".

On this event, the new process should be aware that the share memory might have experienced a exception; therefore, it should be checked.

Once the shared memory and mutex is recovered, you should use "pthread_mutex_consistent()" to return the mutex to a "consistent" state.

the soure code needs to be built with "-lrt" and "-lpthread".

source code:

#include <fcntl.h>
#include <sys/stat.h>
#include <errno.h>
#include <sys/mman.h>
#include <sys/types.h> //shm_open
#include <stdio.h>  //printf
#include <stdlib.h> //exit
#include <unistd.h> //close
#include <string.h> //strerror
#include <pthread.h>//pthread_cond* pthread_mutex*

/* This will be created under /dev/shm/ */
#define STATE_FILE "/program.shared" 

/* Define a struct we wish to share. Notice that we will allocate 
 *  * only sizeof SHARED_VAR, so all sizes are constant              
 *   */
typedef struct
{
 int flags;
 pthread_mutex_t mutex;
 pthread_cond_t cond;
}  SHARED_VAR;

int shared_mutex_init(SHARED_VAR *conf)
{
 pthread_mutexattr_t mutexAttr;
 pthread_mutexattr_init(&mutexAttr);

 do{

  if(pthread_mutexattr_setpshared(&mutexAttr, PTHREAD_PROCESS_SHARED) != 0 ){
   printf("failed to set PTHREAD_PROCESS_SHARED\n");
   break;
  }

  if(pthread_mutexattr_setrobust(&mutexAttr, PTHREAD_MUTEX_ROBUST) != 0){
   printf("failed to set PTHREAD_MUTEX_ROBUST\n");
   break;
  }

  if(pthread_mutex_init(&(conf->mutex), &mutexAttr) != 0){
   printf("failed to init mutex\n");
   break;
  }

  printf("%s success\n", __FUNCTION__);

  return 0;

 }while(0);

 pthread_mutexattr_destroy(&mutexAttr);

 return -1;
}


int shared_cond_init(SHARED_VAR *conf)
{
 pthread_condattr_t condAttr;
 pthread_condattr_init(&condAttr);

 do{

  if(pthread_condattr_setpshared(&condAttr, PTHREAD_PROCESS_SHARED) != 0 ){
   printf("failed to set PTHREAD_PROCESS_SHARED\n");
   break;
  }


  if(pthread_cond_init(&(conf->cond), &condAttr) != 0){
   printf("failed to init mutex\n");
   break;
  }

  printf("%s success\n", __FUNCTION__);

  return 0;

 }while(0);

 pthread_condattr_destroy(&condAttr);

 return -1;
}

int main (void)
{
 int first = 0;
 int i;
 int shm_fd;
 static SHARED_VAR *conf;
 int process_id;
 struct timespec abstime;

 /* Try to open the shm instance with  O_EXCL,
  *    * this tests if the shm is already opened by someone else 
  *       */
 if((shm_fd = shm_open(STATE_FILE, (O_CREAT | O_EXCL | O_RDWR), 
     (S_IREAD | S_IWRITE))) > 0 ) {
  first = 1; /* We are the first instance */
 }
 else if((shm_fd = shm_open(STATE_FILE, (O_CREAT | O_RDWR), 
     (S_IREAD | S_IWRITE))) < 0) {
  /* Try to open the shm instance normally and share it with 
   *     * existing clients 
   *         */
  printf("Could not create shm object. %s\n", strerror(errno));
  return errno;
 } 

 /* Set the size of the SHM to be the size of the struct. */
 ftruncate(shm_fd, sizeof(SHARED_VAR));

 /* Connect the conf pointer to set to the shared memory area,
  *    * with desired permissions 
  *       */
 if((conf =  mmap(0, sizeof(SHARED_VAR), (PROT_READ | PROT_WRITE), 
     MAP_SHARED, shm_fd, 0)) == MAP_FAILED) {

  return errno;

 }
 if(first) {
  /* Run a set up for the first time, fill some args */
  printf("First creation of the shm. Setting up default values\n");
  conf->flags = 0;
 }
 else
 {
  printf("Value of flags = %d\n", conf->flags);
 }
 
 if(shared_mutex_init(conf) != 0){
  printf("failed to create shared_mutex\n");
  return 0;
 }

 if(shared_cond_init(conf) != 0){
  printf("failed to create shared_cond\n");
  return 0;
 }

 
 int lockReturn;
 int consistent;
 int condReturn;

 do{

  lockReturn  = pthread_mutex_lock(&(conf->mutex));

  if(lockReturn ==  EOWNERDEAD){   

   /*Previous process died before releasing the mutex do some cleanup */

   consistent = pthread_mutex_consistent(&(conf->mutex));/*address that the cleanup is done*/
  }else if(lockReturn ! = 0){

   /*didn't get lock*/
   continue;
  }

  if(/*need to wait*/){
   clock_gettime(CLOCK_REALTIME, &abstime);
   abstime.tv_sec += 5;
   condReturn = pthread_cond_timedwait(&(conf->cond), &(conf->mutex), &abstime);
  }

  if(/*need to signal*/){
   pthread_cond_signal(&(conf->cond));
  }

  /*do something*/

  pthread_mutex_unlock(&(conf->mutex));

 }while( /*running()*/);

 close(shm_fd);
 exit(0);
}

Sauron@Mordor

2014年12月10日星期三

Setting up shared memory with robust pthread_mutex and pthread_cond

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2014年12月10日 星期三

Setting up shared memory with robust pthread_mutex and pthread_cond

沒有留言:

2014年12月10日星期三