关于信号量与线程互斥锁的区别与实现

线程互斥锁pthread_mutex_t的实现原理：
pthread_mutex_lock:
atomic_dec(pthread_mutex_t.value);
if(pthread_mutex_t.value!=0)
   futex(WAIT)
else
   success

pthread_mutex_unlock:
atomic_inc(pthread_mutex_t.value);
if(pthread_mutex_t.value!=1)
   futex(WAKEUP)
else
   success

信号量sem_t的实现原理（直接从glibc/nptl/DESIGN-sem.txt中摘的）：
sem_wait(sem_t *sem)
{
  for (;;) {

    if (atomic_decrement_if_positive(sem->count))
      break;

    futex_wait(&sem->count, 0)
  }
}

sem_post(sem_t *sem)
{
  n = atomic_increment(sem->count);
  // Pass the new value of sem->count
  futex_wake(&sem->count, n + 1);