Re: [PATCH v15] kern: simple futex for gnumach

[Marin Ramesa]

On 01/15/2014 10:56:25 AM, Justus Winter wrote:
> > I tested a simple userspace mutex based on this and multiple  
> futexes,
> > all tests passed.
>
> Could you put that somewhere? I still have so little "context", I'd
> like to see how futexe are used in a mutex implementation.

Here's what I used:

#include "gnumach.h"
#include <mach_init.h>
#include <pthread.h>
#include <stdio.h>

struct simple_mutex {

       #define SMUTEX_UNLOCKED 0
       #define SMUTEX_NO_WAITERS 1
       #define SMUTEX_WAITERS 2

       int value;

};

void simple_mutex_init(struct simple_mutex *smutex)
{
       smutex->value = SMUTEX_UNLOCKED;
}

void simple_mutex_lock(struct simple_mutex *smutex)
{
       int c;

       if ((c = __sync_val_compare_and_swap(&smutex->value,  
SMUTEX_UNLOCKED, SMUTEX_NO_WAITERS)) != SMUTEX_UNLOCKED) {
               if (c != SMUTEX_WAITERS)
                       c = __sync_lock_test_and_set(&smutex->value,  
SMUTEX_WAITERS);
               while (c != SMUTEX_UNLOCKED) {
                       futex_wait(mach_task_self(),  
(vm_offset_t)&smutex->value, SMUTEX_WAITERS, 0, 1);
                       c = __sync_lock_test_and_set(&smutex->value,  
SMUTEX_WAITERS);
               }
       }
}

void simple_mutex_unlock(struct simple_mutex *smutex)
{
       if (__sync_lock_test_and_set(&smutex->value, smutex->value - 1)  
!= SMUTEX_NO_WAITERS) {
               smutex->value = SMUTEX_UNLOCKED;
               futex_wake(mach_task_self(),  
(vm_offset_t)&smutex->value, 0, 1);
       }
}

int e = 0;
struct simple_mutex mutex;

void *thread_f(void *arg)
{
        printf("Tread 2 locking the mutex.\n");
        simple_mutex_lock(&mutex);
        printf("Thread 2 in critical section.\n");
        printf("Thread 2 about to sleep.\n");
        futex_wait(mach_task_self(), (vm_offset_t)&e, e, 1000, 1);
        printf("Thread 2 waking up.\n");
        simple_mutex_unlock(&mutex);
        printf("Thread 2 unlocked the mutex.\n");
        return NULL;
}

int main()
{
        simple_mutex_init(&mutex);

        pthread_t new_thread;
        pthread_attr_t attr;

        pthread_attr_init(&attr);

        pthread_create(&new_thread, &attr, thread_f, NULL);

        futex_wait(mach_task_self(), (vm_offset_t)&e, e, 1000, 1);

        printf("Thread 1 trying to lock the mutex.\n");
        simple_mutex_lock(&mutex);
        printf("Thread 1 in critical section.\n");
        simple_mutex_unlock(&mutex);
        printf("Thread 1 unlocked the mutex.\n");

        return 0;
}