Subject: RE: futex priority based wakeup

From: "Ilya Lipovsky" <[email-masked]>
Date: Fri, 7 Sep 2007 12:41:12 -0400

Looks like it is a libc issue. Apparently, GNU libc supports priority
futexes only in version 2.5 and higher.

-----Original Message-----
From: linuxppc-embedded-bounces+lipovsky=[email-masked]
[mailto:linuxppc-embedded-bounces+lipovsky=[email-masked]] On Behalf
Of Benedict, Michael
Sent: Friday, September 07, 2007 11:02 AM
To: [email-masked]
Subject: futex priority based wakeup

I recently upgraded to 2.6.22 to get the priority based futex wakeup
behavior.  However, when I run a simple program (see below), based on
either a pthread_mutex_t or a sem_t, it seems that threads are woken up
in FIFO order.  I am using glibc 2.3.6 with NPTL and TLS, based off
crossdev-0.43.  Could someone help me get priority-based wakeup or point
me to a better place to get this question answered?
	Thank you,
		Michael

Code:

pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;

void *important(void *ign)
{
        sleep(1);
        printf("important waiting for mutex\n");
        if(pthread_mutex_lock(&mymutex)) {
                perror("sem_wait");
                exit(1);
        } else {
                printf("important got mutex!\n");
                pthread_mutex_unlock(&mymutex);
        }

        return NULL;
}

void *unimportant(void *ign)
{
        printf("unimportant waiting for mutex\n");
        if(pthread_mutex_lock(&mymutex)) {
                perror("sem_wait");
                exit(1);
        } else {
                printf("unimportant got mutex!\n");
                pthread_mutex_unlock(&mymutex);
        }

        return NULL;
}

int main()
{
        struct sched_param p;
        pthread_attr_t attr;
        pthread_t i, u;

        pthread_mutex_lock(&mymutex);

        p.__sched_priority = sched_get_priority_min(SCHED_FIFO);
        if(-1 == p.__sched_priority) {
                perror("sched_get_priority_min");
                return 1;
        }
        pthread_attr_init(&attr);
        pthread_attr_setschedpolicy(&attr, SCHED_FIFO);
        pthread_attr_setschedparam(&attr, &p);
        pthread_create(&u, &attr, unimportant, NULL);

        p.__sched_priority = sched_get_priority_max(SCHED_FIFO);
        pthread_attr_setschedparam(&attr, &p);
        pthread_create(&i, &attr, important, NULL);

        sleep(5);
        printf("main unlocking mutex\n");
        pthread_mutex_unlock(&mymutex);

        pthread_join(u, NULL);
        pthread_join(i, NULL);

        return 0;
}

Which produces:
unimportant waiting for mutex
important waiting for mutex
main unlocking mutex
unimportant got mutex!
important got mutex!

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