diff --git a/docs/kernel/locking/index.rst b/docs/kernel/locking/index.rst index 9c4f54a2..227bd421 100644 --- a/docs/kernel/locking/index.rst +++ b/docs/kernel/locking/index.rst @@ -11,4 +11,5 @@ spinlock lockref mutex + rwlock diff --git a/docs/kernel/locking/rwlock.md b/docs/kernel/locking/rwlock.md new file mode 100644 index 00000000..384ab228 --- /dev/null +++ b/docs/kernel/locking/rwlock.md @@ -0,0 +1,204 @@ +# RwLock读写锁 +:::{note} +本文作者: sujintao + +Email: +::: + +## 1. 简介 +  读写锁是一种在并发环境下保护多进程间共享数据的机制. 相比于普通的spinlock,读写锁将对 +共享数据的访问分为读和写两种类型: 只读取共享数据的访问使用读锁控制,修改共享数据的访问使用 +写锁控制. 读写锁设计允许同时存在多个"读者"(只读取共享数据的访问)和一个"写者"(修改共享数据 +的访问), 对于一些大部分情况都是读访问的共享数据来说,使用读写锁控制访问可以一定程度上提升性能. + +## 2. DragonOS中读写锁的实现 +### 2.1 读写锁的机理 +  读写锁的目的是维护多线程系统中的共享变量的一致性. 数据会被包裹在一个RwLock的数据结构中, 一切的访问必须通过RwLock的数据结构进行访问和修改. 每个要访问共享数据的会获得一个守卫(guard), 只读进程获得READER(读者守卫),需要修改共享变量的进程获得WRITER(写者守卫),作为RwLock的"影子", 线程都根据guard来进行访问和修改操作. + +  在实践中, 读写锁除了READER, WRITER, 还增加了UPGRADER; 这是一种介于READER和WRITER之间的守卫, 这个守卫的作用就是防止WRITER的饿死(Staration).当进程获得UPGRADER时,进程把它当成READER来使用;但是UPGRADER可以进行升级处理,升级后的UPGRADER相当于是一个WRITER守卫,可以对共享数据执行写操作. + +  所有守卫都满足rust原生的RAII机理,当守卫所在的作用域结束时,守卫将自动释放. + +### 2.2 读写锁守卫之间的关系 +  同一时间点, 可以存在多个READER, 即可以同时有多个进程对共享数据进行访问;同一时间只能存在一个WRITER,而且当有一个进程获得WRITER时,不能存在READER和UPGRADER;进程获得UPGRADER的前提条件是,不能有UPGRADER或WRITER存在,但是当有一个进程获得UPGRADER时,进程无法成功申请READER. + +### 2.3 设计的细节 + +#### 2.3.1 RwLock数据结构 +```rust +pub struct RwLock { + lock: AtomicU32,//原子变量 + data: UnsafeCell, +} +``` +#### 2.3.2 READER守卫的数据结构 +```rust +pub struct RwLockReadGuard<'a, T: 'a> { + data: *const T, + lock: &'a AtomicU32, +} +``` + +#### 2.3.3 UPGRADER守卫的数据结构 +```rust +pub struct RwLockUpgradableGuard<'a, T: 'a> { + data: *const T, + inner: &'a RwLock, +} +``` + +#### 2.3.4 WRITER守卫的数据结构 +```rust +pub struct RwLockWriteGuard<'a, T: 'a> { + data: *mut T, + inner: &'a RwLock, +} +``` + +#### 2.3.5 RwLock的lock的结构介绍 +lock是一个32位原子变量AtomicU32, 它的比特位分配如下: +``` + UPGRADER_BIT WRITER_BIT + ^ ^ +OVERFLOW_BIT +------+ +-------+ + ^ | | + | | | ++-+--+--------------------------------------------------------+-+--+-+--+ +| | | | | +| | | | | +| | The number of the readers | | | +| | | | | ++----+--------------------------------------------------------+----+----+ + 31 30 2 1 0 +``` + +  (从右到左)第0位表征WRITER是否有效,若WRITER_BIT=1, 则存在一个进程获得了WRITER守卫; 若UPGRADER_BIT=1, 则存在一个进程获得了UPGRADER守卫,第2位到第30位用来二进制表示获得READER守卫的进程数; 第31位是溢出判断位, 若OVERFLOW_BIT=1, 则不再接受新的读者守卫的获得申请. + + +## 3. 读写锁的主要API +### 3.1 RwLock的主要API +```rust +///功能: 输入需要保护的数据类型data,返回一个新的RwLock类型. +pub const fn new(data: T) -> Self +``` +```rust +///功能: 获得READER守卫 +pub fn read(&self) -> RwLockReadGuard +``` +```rust +///功能: 尝试获得READER守卫 +pub fn try_read(&self) -> Option> +``` +```rust +///功能: 获得WRITER守卫 +pub fn write(&self) -> RwLockWriteGuard +``` +```rust +///功能: 尝试获得WRITER守卫 +pub fn try_write(&self) -> Option> +``` +```rust +///功能: 获得UPGRADER守卫 +pub fn upgradeable_read(&self) -> RwLockUpgradableGuard +``` +```rust +///功能: 尝试获得UPGRADER守卫 +pub fn try_upgradeable_read(&self) -> Option> +``` +### 3.2 WRITER守卫RwLockWriteGuard的主要API +```rust +///功能: 将WRITER降级为READER +pub fn downgrade(self) -> RwLockReadGuard<'rwlock, T> +``` +```rust +///功能: 将WRITER降级为UPGRADER +pub fn downgrade_to_upgradeable(self) -> RwLockUpgradableGuard<'rwlock, T> +``` +### 3.3 UPGRADER守卫RwLockUpgradableGuard的主要API +```rust +///功能: 将UPGRADER升级为WRITER +pub fn upgrade(mut self) -> RwLockWriteGuard<'rwlock, T> +``` +```rust +///功能: 将UPGRADER降级为READER +pub fn downgrade(self) -> RwLockReadGuard<'rwlock, T> +``` + +## 4. 用法实例 +```rust +static LOCK: RwLock = RwLock::new(100 as u32); + +fn t_read1() { + let guard = LOCK.read(); + let value = *guard; + let readers_current = LOCK.reader_count(); + let writers_current = LOCK.writer_count(); + println!( + "Reader1: the value is {value} + There are totally {writers_current} writers, {readers_current} readers" + ); +} + +fn t_read2() { + let guard = LOCK.read(); + let value = *guard; + let readers_current = LOCK.reader_count(); + let writers_current = LOCK.writer_count(); + println!( + "Reader2: the value is {value} + There are totally {writers_current} writers, {readers_current} readers" + ); +} + +fn t_write() { + let mut guard = LOCK.write(); + *guard += 100; + let writers_current = LOCK.writer_count(); + let readers_current = LOCK.reader_count(); + println!( + "Writers: the value is {guard} + There are totally {writers_current} writers, {readers_current} readers", + guard = *guard + ); + let read_guard=guard.downgrade(); + let value=*read_guard; + println!("After downgraded to read_guard: {value}"); +} + +fn t_upgrade() { + let guard = LOCK.upgradeable_read(); + let value = *guard; + let readers_current = LOCK.reader_count(); + let writers_current = LOCK.writer_count(); + println!( + "Upgrader1 before upgrade: the value is {value} + There are totally {writers_current} writers, {readers_current} readers" + ); + let mut upgraded_guard = guard.upgrade(); + *upgraded_guard += 100; + let writers_current = LOCK.writer_count(); + let readers_current = LOCK.reader_count(); + println!( + "Upgrader1 after upgrade: the value is {temp} + There are totally {writers_current} writers, {readers_current} readers", + temp = *upgraded_guard + ); + let downgraded_guard=upgraded_guard.downgrade_to_upgradeable(); + let value=*downgraded_guard; + println!("value after downgraded: {value}"); + let read_guard=downgraded_guard.downgrade(); + let value_=*read_guard; + println!("value after downgraded to read_guard: {value_}"); +} + +fn main() { + let r2=thread::spawn(t_read2); + let r1 = thread::spawn(t_read1); + let t1 = thread::spawn(t_write); + let g1 = thread::spawn(t_upgrade); + r1.join().expect("r1"); + t1.join().expect("t1"); + g1.join().expect("g1"); + r2.join().expect("r2"); +} +``` \ No newline at end of file