How to modify structure elements atomically without using locks in C?

Question

I would like to modify some elements of a structure atomically. My current implementation uses mutexes to protect the critical code, and can be seen below.

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <pthread.h>

pthread_mutex_t thread_mutex = PTHREAD_MUTEX_INITIALIZER;

#define ITER    100000

typedef struct global_status {
    int32_t context_delta;
    uint32_t global_access_count;
} global_status_t;

global_status_t g_status;

void *context0(void *ptr)
{
    unsigned int iter = ITER;
    while (iter--) {
        wait_event_from_device0();
        pthread_mutex_lock(&thread_mutex);
        g_status.context_delta++;
        g_status.global_access_count++;
        pthread_mutex_unlock(&thread_mutex);
    }

    return NULL;
}

void *context1(void *ptr)
{
    unsigned int iter = ITER;
    while (iter--) {
        wait_event_from_device1();
        pthread_mutex_lock(&thread_mutex);
        g_status.context_delta--;
        g_status.global_access_count++;
        pthread_mutex_unlock(&thread_mutex);
    }

    return NULL;
}

int main(int argc, char **argv)
{
    pthread_t tid0, tid1;
    int iret;

    if ((iret = pthread_create(&tid0, NULL, context0, NULL))) {
         fprintf(stderr, "context0 creation error!\n");
         return EXIT_FAILURE;
    }

    if ((iret = pthread_create(&tid1, NULL, context1, NULL))) {
         fprintf(stderr, "context1 creation error!\n");
         return EXIT_FAILURE;
    }

    pthread_join(tid0, NULL);
    pthread_join(tid1, NULL);

    printf("%d, %d\n", g_status.context_delta, g_status.global_access_count);
    return 0;
}

I am planning to port this code into an RTOS which does not support posix, and I would like to do this operation atomically without using mutexes or disabling/enabling interrupts.

How can I do this operation? Is it possible by using 'atomic compare and swap function' (CAS)?

Answer 1

Seems like in your example you have two threads servicing to different devices. You maybe able to do away with locking completely using a per-device structure. The global will be the aggregate of all per-device statistics. If you do need locks you can use CAS, LL/SC or any supported underlying atomic construct.

Answer 2

What i do is create a union with all the fields I want to change at the same time. like this:

union {
  struct {
    int            m_field1;
    unsigned short m_field2 : 2,
                   m_field3 : 1;
    BYTE           m_field4; 
  }
  unsigned long long m_n64;
  TData(const TData& r) { m_n64 = r.m_n64; }
} TData;

You embed unions like that inside your larger struct like this:

struct {
  ...
  volatile TData m_Data;
  ...
} TBiggerStruct;

Then i do something like this:

while (1) {
  TData Old = BiggerSharedStruct.m_Data, New = Old;
  New.field1++;
  New.field4--;
  if (CAS(&SharedData.m_n64, Old.m_n64, New.m_n64))
    break; // success
}

I do a lot of packing of fields that I want to change at the same time into the smallest possible 16, 32, or 64 bit structure. I think 128 bit stuff on intel is not as fast as the 64 bit stuff, so I avoid it. I haven't benchmarked it in awhile so I could be wrong on that.