第5章：并发与竞态条件-11：Reader/Writer Spinlocks

In continuation of the previous text 第5章：并发与竞态条件-10：The Spinlock Functions, let's GO ahead.

Reader/Writer Spinlocks

The kernel provides a reader/writer form of spinlocks that is directly analogous to the reader/writer semaphores we saw earlier in this chapter. These locks allow any number of readers into a critical section simultaneously, but writers must have exclusive access. Reader/writer locks have a type of rwlock_t, defined in . They can be declared and initialized in two ways:

内核提供了一种读写形式的自旋锁，其功能与本章前文介绍的读写信号量（rwsem）直接类似。这类锁允许任意数量的读线程同时进入临界区，但写线程必须独占访问。读写自旋锁的类型为 rwlock_t，定义在 <linux/spinlock.h> 头文件中。它们可通过以下两种方式声明和初始化：

rwlock_t my_rwlock = RW_LOCK_UNLOCKED; /* Static way */
rwlock_t my_rwlock;
rwlock_init(&my_rwlock); /* Dynamic way */

The list of functions available should lookreasonably familiar by now. For readers, the following functions are available:

目前可用的函数列表应该看起来相当熟悉了。读线程可使用以下函数：

void read_lock(rwlock_t *lock);
void read_lock_irqsave(rwlock_t *lock, unsigned long flags);
void read_lock_irq(rwlock_t *lock);
void read_lock_bh(rwlock_t *lock);
void read_unlock(rwlock_t *lock);
void read_unlock_irqrestore(rwlock_t *lock, unsigned long flags);
void read_unlock_irq(rwlock_t *lock);
void read_unlock_bh(rwlock_t *lock);

Interestingly, there is no read_trylock. The functions for write access are similar:

有趣的是，读写自旋锁没有 read_trylock（读线程非阻塞尝试获取锁）函数。写线程的访问函数与之类似：

void write_lock(rwlock_t *lock);
void write_lock_irqsave(rwlock_t *lock, unsigned long flags);
void write_lock_irq(rwlock_t *lock);
void write_lock_bh(rwlock_t *lock);
int write_trylock(rwlock_t *lock);
void write_unlock(rwlock_t *lock);
void write_unlock_irqrestore(rwlock_t *lock, unsigned long flags);
void write_unlock_irq(rwlock_t *lock);
void write_unlock_bh(rwlock_t *lock);

Reader/writer locks can starve readers just as rwsems can. This behavior is rarely a problem; however, if there is enough lockcontention to bring about starvation, performance is poor anyway.

与读写信号量一样，读写自旋锁也可能导致读线程饥饿（即读线程长时间无法获取锁）。但这种情况通常不会构成严重问题：如果锁的竞争激烈到足以引发饥饿，系统性能本身就已经很差了。

补充说明：

1. 函数核心语义

   读写自旋锁的函数用法与普通自旋锁完全一致，仅区分 “读访问” 和 “写访问”—— 读函数允许多读者并发，写函数保证单写者独占，且读、写操作互斥。

2. 配对使用要求

   必须严格遵循 “获取 - 释放” 函数配对原则，例如 read_lock_irqsave 需与 read_unlock_irqrestore 配对，write_lock_bh 需与 write_unlock_bh 配对，避免中断或软中断状态异常。

3. 无 read_trylock 的设计考量

   内核认为读线程非阻塞获取锁的需求极少，且实现该功能需处理多读者并发的原子性判断，复杂度高且收益有限，因此未提供该接口。

4. 适用场景限制 

   ​​​​​​​仅适用于 “读操作远多于写操作、临界区极短” 的场景，且支持中断上下文、软中断上下文与进程上下文的混合使用（需通过对应带中断 / 软中断禁用的函数）。

5. 饥饿问题补充

   ​​​​​​​​​​​​​​读线程饥饿的本质是 “写线程优先级高于读线程”—— 当有写线程等待时，新的读线程会被阻塞，直到所有等待的写线程完成操作。若写操作频繁，会导致读线程长期无法获取锁，此时应改用普通自旋锁。