futex priority based wakeup

[Benedict, Michael]

Ilya Lipovsky wrote:
> Your code looks correct to me, so if the kernel developers
> did their job
> correctly, the only potentially weak link is glibc.
> 

Well, either the kernel developers didn't do their job, or I am missing
something.  The following also fails, and it should be bypassing glibc:

#define _XOPEN_SOURCE 600

#include <linux/futex.h>
#include <sys/time.h>
#include <asm/atomic.h>

#include <sys/syscall.h>
#include <unistd.h>

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

int myfutex = 0;

void *important(void *ign)
{
        sleep(1);
        printf("important waiting for futex\n");
        fflush(stdout);
        if(syscall(SYS_futex, &myfutex, FUTEX_WAIT, 0, NULL)) {
                perror("futex");
                exit(1);
        } else {
                printf("important got futex!\n");
                fflush(stdout);
                syscall(SYS_futex, &myfutex, FUTEX_WAKE, 1, NULL);
}

        return NULL;
}


void *unimportant(void *ign)
{
        printf("unimportant waiting for futex\n");
        fflush(stdout);
        if(syscall(SYS_futex, &myfutex, FUTEX_WAIT, 0, NULL)) {
                perror("futex");
                exit(1);
        } else {
                printf("unimportant got futex!\n");
                fflush(stdout);
                syscall(SYS_futex, &myfutex, FUTEX_WAKE, 1, NULL);
}

        return NULL;
}

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

        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("futex FUTEX_WAKE\n");
        fflush(stdout);
        syscall(SYS_futex, &myfutex, FUTEX_WAKE, 1, NULL);

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

        return 0;
}

Which produces:
unimportant waiting for futex
important waiting for futex
futex FUTEX_WAKE
unimportant got futex!
important got futex!


Could someone with 2.6.22 please verify?