Как Futex работает в моем коде?

Question

Я вижу беспорядочный вывод из моего кода futex.

Вот мой код,

#include <stdio.h>
#include <pthread.h>
#include <linux/futex.h>
#include <syscall.h>
#include <unistd.h>
#include <limits.h>
#include <time.h>
#include <sys/time.h>
#define NUM 15

int futex_addr=1;

int futex_wait(int* addr, int val1){

  if( --(*addr)==0)
  {
    printf(" after dec :futex : %d\n",futex_addr);
    return 0;
  }
  else
  {
  *addr==-1;
  return syscall(SYS_futex,&futex_addr,val1, NULL, NULL, 0);
  }
}
int futex_wake(void* addr, int n){
  return syscall(SYS_futex, addr, FUTEX_WAKE, n, NULL, NULL, 0);
}

void* thread_f(void* par){
        int id = (int) par;

    /*go to sleep*/
    /*futex_addr = 0;*/
        //printf("before wait, futex : %d\n",futex_addr);

    futex_wait(&futex_addr,0);

    printf("after wait, futex : %d\n",futex_addr);
    /*if((--futex_addr)!=0)
    {
     futex_addr=-1;
     futex_wait(&futex_addr,0);
    }

    else
    {*/
        printf("Thread %d starting to work!\n",id);

    for (int i=0; i<30000; i++)
    {
        //printf(" Thread %d is in loop for %dth time\n",id,i);
    }
    printf(" thread %d finished\n",id);

    if(++futex_addr != 1)
    {
        futex_addr=1;
            futex_wake(&futex_addr,INT_MAX);
    }   
    return NULL;
    /*}*/ 
}

int main(){
        pthread_t threads[NUM];
        int i;

    struct timeval start,end,elapsed;

    gettimeofday(&start, NULL);

        for (i=0;i<NUM;i++){
                pthread_create(&threads[i],NULL,thread_f,(void *)i);
        }

        //printf("Everyone wait...\n");
        sleep(1);
        //printf("Now go!\n");
    /*wake threads*/

    futex_wake(&futex_addr,INT_MAX);

    /*give the threads time to complete their tasks*/
        //sleep(18);
    for (int i = 0; i < NUM; i++) {
    pthread_join(threads[i], NULL); 
   }
    printf("Main is quitting...\n");

    gettimeofday(&end, NULL);
    timersub(&end, &start, &elapsed);

    printf("secs: %ld.%06ld seconds\n msecs: %ld.%06ld seconds\n",
            (long)(elapsed.tv_sec), (long)(elapsed.tv_usec));


        return 0;
}

выход:

 after dec :futex : 0
after wait, futex : 0
Thread 0 starting to work!
 thread 0 finished
 after dec :futex : 0
after wait, futex : 0
Thread 1 starting to work!
 thread 1 finished
 after dec :futex : 0
after wait, futex : 0
Thread 2 starting to work!
after wait, futex : -2
Thread 4 starting to work!
 thread 4 finished
after wait, futex : -3
Thread 5 starting to work!
 thread 2 finished
 thread 5 finished
after wait, futex : -4
Thread 6 starting to work!
 thread 6 finished
after wait, futex : -5
Thread 7 starting to work!
 thread 7 finished
after wait, futex : -6
Thread 8 starting to work!
 thread 8 finished
after wait, futex : -7
Thread 9 starting to work!
 thread 9 finished
after wait, futex : -8
Thread 10 starting to work!
 thread 10 finished
after wait, futex : -9
Thread 11 starting to work!
 thread 11 finished
after wait, futex : -10
Thread 12 starting to work!
 thread 12 finished
after wait, futex : -11
Thread 13 starting to work!
 thread 13 finished
after wait, futex : -12
after wait, futex : -1
Thread 3 starting to work!
 thread 3 finished
Thread 14 starting to work!
 thread 14 finished
Main is quitting...
secs: 1.001813 seconds
 msecs: 14.134513484 seconds

Проблема здесь в том, что перед тем, как поток 2 освободит futex, другой поток получит его (если вы видите в выводе tht).

 Thread 2 starting to work!
    after wait, futex : -2
    Thread 4 starting to work!
     thread 4 finished
    after wait, futex : -3
    Thread 5 starting to work!
     thread 2 finished

Так почему же так происходит? когда поток 2 использует futex, как он передается потоку 4? Есть ли какой-либо недостаток в коде?