Go为什么不支持可重入锁呢？

先来看看什么是可重入锁，以下是维基百科的定义。

计算机科学中，可重入互斥锁（英语：reentrant mutex）是互斥锁的一种，同一线程对其多次加锁不会产生死锁。可重入互斥锁也称递归互斥锁（英语：recursive mutex）或递归锁（英语：recursive lock）。

如果对已经上锁的普通互斥锁进行“加锁”操作，其结果要么失败，要么会阻塞至解锁。而如果换作可重入互斥锁，当且仅当尝试加锁的线程就是持有该锁的线程时，类似的加锁操作就会成功。可重入互斥锁一般都会记录被加锁的次数，只有执行相同次数的解锁操作才会真正解锁。

递归互斥锁解决了普通互斥锁不可重入的问题：如果函数先持有锁，然后执行回调，但回调的内容是调用它自己，就会产生死锁。[1]

Go里面是不支持可重入互斥锁的。看代码：

var mu sync.Mutex
func main() {
	mu.Lock()
	mu.Lock()
}

运行后会报

fatal error: all goroutines are asleep - deadlock!

Go 设计原则

在工程中使用互斥的根本原因是：为了保护不变量，也可以用于保护内、外部的不变量。

基于此，Go 在互斥锁设计上会遵守这几个原则。如下：

  • 在调用 mutex.Lock 方法时，要保证这些变量的不变性保持，不会在后续的过程中被破坏。
  • 在调用 mu.Unlock 方法时，要保证：
    • 程序不再需要依赖那些不变量。
    • 如果程序在互斥锁加锁期间破坏了它们，则需要确保已经恢复了它们。

不支持的原因

讲了 Go 自己的设计原则后，那为什么不支持可重入呢？

其实 Russ Cox 于 2010 年在《Experimenting with GO》就给出了答复，认为递归（又称：重入）互斥是个坏主意，这个设计并不好。

总结

Go 互斥锁没有支持可重入锁的设计，这样就不用关心递归层级等复杂的流程了。加锁解锁中间做了什么一目了然。

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