linux-kernel - 关于 linux 内核中的信号量 up() 和 mutex_unlock()

Question

我想知道为什么我们可以在中断上下文中使用信号量 up() 而互斥量的相同变体，即 mutex_unlock() 不能在中断上下文中使用。以下是内核的片段

/**
* mutex_unlock - release the mutex
* @lock: the mutex to be released
*
* Unlock a mutex that has been locked by this task previously.
*
* This function must not be used in interrupt context. Unlocking
* of a not locked mutex is not allowed.
*
* This function is similar to (but not equivalent to) up().
*/
void __sched mutex_unlock(struct mutex *lock)
{
  #ifndef CONFIG_DEBUG_LOCK_ALLOC
  if (__mutex_unlock_fast(lock))
    return;
#endif
__mutex_unlock_slowpath(lock, _RET_IP_);
}EXPORT_SYMBOL(mutex_unlock);


 /**
 * up - release the semaphore
* @sem: the semaphore to release
*
* Release the semaphore.  Unlike mutexes, up() may be called from any
* context and even by tasks which have never called down().
*/
void up(struct semaphore *sem)
{
  unsigned long flags;

  raw_spin_lock_irqsave(&sem->lock, flags);
  if (likely(list_empty(&sem->wait_list)))
    sem->count++;
  else
     __up(sem);
  raw_spin_unlock_irqrestore(&sem->lock, flags);
}
EXPORT_SYMBOL(up);

Answer 1

mutex_unlock 需要一个 mutex-internal spinlock in a non-irq-safefashion - 例如如果设置了 CONFIG_PREEMPT_RT，raw_spin_lock 可能会休眠 - 因此，如果时机正确，IRQ 上下文中的 mutex_unlock 将死锁。