The code for our reader-writer lock from class/the textbook is shown below. What is the main disadvantage of this code?

typedef struct _rwlock_t {
  sem_t lock; 	// binary semaphore (basic lock)
  sem_t writelock; 	// used to allow ONE writer or MANY readers
  int readers; 	// count of readers reading in critical section
} rwlock_t;
void rwlock_init(rwlock_t *rw) {
  rw->readers = 0;
  sem_init(&rw->lock, 0, 1);
  sem_init(&rw->writelock, 0, 1);
}
void rwlock_acquire_readlock(rwlock_t *rw) {
  sem_wait(&rw->lock);
  rw->readers++;
  if (rw->readers == 1)
    sem_wait(&rw->writelock); // first reader acquires writelock
  sem_post(&rw->lock);
}
void rwlock_release_readlock(rwlock_t *rw) {
  sem_wait(&rw->lock);
  rw->readers--;
  if (rw->readers == 0)
    sem_post(&rw->writelock); // last reader releases writelock
  sem_post(&rw->lock);
}
void rwlock_acquire_writelock(rwlock_t *rw) {
  sem_wait(&rw->writelock);
}
void rwlock_release_writelock(rwlock_t *rw) {
  sem_post(&rw->writelock);
}