My recursive mutex vs pthread_mutex_t (type: recursive) (repost, push)

问题

I was wondering if I could make a recursive mutex type on my own with a PTHREAD_MUTEX_ERRORCHECK mutex, this is the result:

typedef struct {
    pthread_mutex_t mutex;
    uint32_t deadlocks;
} pthread_recursivemutex_t;

int pthread_recursivemutex_init(pthread_recursivemutex_t *mutex)
{
    int ret;
    pthread_mutexattr_t attr;

    mutex->deadlocks = 0;

    ret = pthread_mutexattr_init(&attr);

    if (ret != 0) {
        return ret;
    }

    (void)pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK);

    ret = pthread_mutex_init(&mutex->mutex, &attr);

    (void)pthread_mutexattr_destroy(&attr);

    return ret;
}

void pthread_recursivemutex_lock(pthread_recursivemutex_t *mutex)
{
    int ret;    

    ret = pthread_mutex_lock(&mutex->mutex);

    if (ret == 0) {
        mutex->deadlocks = 0;
    } else if (ret == EDEADLK) {
        mutex->deadlocks += 1;
    }
}

void pthread_recursivemutex_unlock(pthread_recursivemutex_t *mutex)
{
    if (mutex->deadlocks == 0) {
        (void)pthread_mutex_unlock(&mutex->mutex);
    } else {
        mutex->deadlocks -= 1;
    }
}

void pthread_recursivemutex_destroy(pthread_recursivemutex_t *mutex)
{
    (void)pthread_mutex_destroy(&mutex->mutex);
}

I found out that this type of recursive mutex is a lot faster than a mutex with the PTHREAD_MUTEX_RECURSIVE attribute:

iterations               : 1000000

pthread_mutex_t          : 71757 μSeconds
pthread_recursivemutex_t : 48583 μSeconds

Test code (each called 1000000 times):

void mutex_test()
{
    pthread_mutex_lock(&recursiveMutex);
    pthread_mutex_lock(&recursiveMutex);
    pthread_mutex_unlock(&recursiveMutex);
    pthread_mutex_unlock(&recursiveMutex);
}

void recursivemutex_test()
{
    pthread_recursivemutex_lock(&myMutex);
    pthread_recursivemutex_lock(&myMutex);
    pthread_recursivemutex_unlock(&myMutex);
    pthread_recursivemutex_unlock(&myMutex);
}

pthread_recursivemutex_t is almost twice as fast as pthread_mutex_t ?! But both behave the same way...?

BTW:

pthread_mutex_t my_mutex;

void *m(void*a)
{
    printf("m: %d %d\n", pthread_mutex_lock(&my_mutex), EDEADLK);
    usleep(500 * 1000);
    printf("m: %d\n", pthread_mutex_lock(&my_mutex));
    pthread_mutex_unlock(&my_mutex);

    return a;
}

void *m2(void *a)
{
    printf("UNLOCKED!!: %d\n", pthread_mutex_lock(&my_mutex));
    usleep(500 * 1000);
    printf("m2: %d\n", pthread_mutex_lock(&my_mutex));

    return a;
}

int main(int argc, const char * argv[])
{
    pthread_t t,t2;
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK);
    pthread_mutex_init(&my_mutex, &attr);

    pthread_create(&t, NULL, m, NULL);
    pthread_create(&t2, NULL, m2, NULL);

    pthread_join(t, NULL);
    pthread_join(t2, NULL);

    return 0;
}

Gives

m: 0 11 m: 11 UNLOCKED!!: 0 m2: 11

Which means thread 2 has no chance to get the mutex while its locked.

来源：https://stackoverflow.com/questions/24856103/my-recursive-mutex-vs-pthread-mutex-t-type-recursive-repost-push

标签

recursion

pthreads

mutex

recursive-mutex