Pthread reader writer locks can starve multiple writers

// check if write locks are fair
#include <stdio.h>
#include <assert.h>
#include <unistd.h>
#include <pthread.h>

pthread_rwlock_t rwlock;
volatile int killed = 0;

static void *t1_func(void *arg) {
    int i;
    for (i = 0; !killed; i++) {
        int r;
        r = pthread_rwlock_wrlock(&rwlock); assert(r == 0);
        usleep(10000);
        r = pthread_rwlock_unlock(&rwlock); assert(r == 0);
    }
    printf("%lu %d\n", pthread_self(), i);
    return arg;
}

int main(void) {
    int r;

    r = pthread_rwlock_init(&rwlock, NULL); assert(r == 0);
    
    const int nthreads = 2;
    pthread_t tids[nthreads];
    for (int i = 0; i < nthreads; i++) {
        r = pthread_create(&tids[i], NULL, t1_func, NULL); assert(r == 0);
    }
    sleep(10);
    killed = 1;
    for (int i = 0; i < nthreads; i++) {
        void *ret;
        r = pthread_join(tids[i], &ret); assert(r == 0);
    }
    return 0;
}

When executing the above program, one thread gets all of the iterations because the reader writer lock is unfair to multiple writers.

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