zhasha@wirbelwind /home/zhasha ; cat lock.c
#define _GNU_SOURCE
#include <stdbool.h>
#include <unistd.h>
#include <errno.h>
#include <time.h>
#include <sched.h>
#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>

typedef unsigned int uint;
typedef struct _LOCK { uint num; pid_t tid; } LOCK;

void LockSiData( LOCK *shared )
{
        pid_t tid = gettid();
        uint const yield = 4;
        uint i;
        /*
        struct timespec ts = {0};
        ts.tv_nsec = 1;
        */

        while ( shared->tid != tid )
        {
                if ( !(shared->tid) )
                        shared->tid = tid;
                //clock_nanosleep(CLOCK_MONOTONIC, 0, &ts, 0);
                for ( i = 0; i < yield; ++i )
                        sched_yield();
        }

        shared->num++;
}

void FreeSiData( LOCK *shared )
{
        pid_t tid = gettid();
        if ( shared->tid != tid )
                return;
        shared->num--;
        if ( shared->num )
                return;
        shared->tid = (pid_t)0;
        //sched_yield();
}

volatile uint quit = 0;
volatile uint data = 0;
LOCK lock = {0};

void* thread( void *ud )
{
        uint i;
        struct timespec ts = {0};
        ts.tv_nsec = 1;
        (void)ud;

        while ( quit < CLOCKS_PER_SEC )
        {
                LockSiData( &lock );

                i = data++;
                if (i != 0)
                {
                        flockfile( stdout );
                        printf("var = %u, expected 0\n", i);
                        funlockfile( stdout );
                        FreeSiData( &lock );
                        exit(1);
                }

                /* Why is needed for the test? */
                clock_nanosleep(CLOCK_MONOTONIC, 0, &ts, 0);

                i = data--;
                if (i != 1)
                {
                        flockfile( stdout );
                        printf("var = %u, expected 1\n", i);
                        funlockfile( stdout );
                        FreeSiData( &lock );
                        exit(1);
                }

                ++quit;

                FreeSiData( &lock );
        }
        return ud;
}

int main()
{
        pthread_t pt;
        int i;

        for (i = 0; i < 2; i++)
        {
                if ((errno = pthread_create(&pt, 0, thread, 0)) != 0 )
                {
                        flockfile( stdout );
                        printf("pthread_create failed: %m\n");
                        funlockfile( stdout );
                        return 1;
                }
        }
        pthread_exit(0);
}
zhasha@wirbelwind /home/zhasha ; gcc -lpthread -O3 lock.c
zhasha@wirbelwind /home/zhasha ; ./a.out
var = 1, expected 0
zhasha@wirbelwind /home/zhasha ; ./a.out
var = 1, expected 0
var = 2, expected 1
zhasha@wirbelwind /home/zhasha ;