golang读写锁RWMutex

读写锁是针对读写的互斥锁

基本遵循两大原则：

1、可以随便读，多个goroutine同时读

2、写的时候，啥也不能干。不能读也不能写

RWMutex提供了四个方法：

func (*RWMutex) Lock // 写锁定

func (*RWMutex) Unlock // 写解锁

func (*RWMutex) RLock // 读锁定

func (*RWMutex) RUnlock // 读解锁

一、随便读

package main

import (
    "sync"
    "time"
)

var m *sync.RWMutex

func main() {
    m = new(sync.RWMutex)
    
    // 多个同时读
    go read(1)
    go read(2)

    time.Sleep(2*time.Second)
}

func read(i int) {
    println(i,"read start")

    m.RLock()
    println(i,"reading")
    time.Sleep(1*time.Second)
    m.RUnlock()    

    println(i,"read over")
}

1 read start

1 reading 

2 read start

2 reading

1 read over

2 read over

可以看出1 读还没有结束，2已经在读

二、写的时候啥也不能干

package main

import (
    "sync"
    "time"
)

var m *sync.RWMutex

func main() {
    m = new(sync.RWMutex)
    
    // 写的时候啥也不能干
    go write(1)
    go read(2)
    go write(3)

    time.Sleep(2*time.Second)
}

func read(i int) {
    println(i,"read start")

    m.RLock()
    println(i,"reading")
    time.Sleep(1*time.Second)
    m.RUnlock()    

    println(i,"read over")
}

func write(i int) {
    println(i,"write start")

    m.Lock()
    println(i,"writing")
    time.Sleep(1*time.Second)
    m.Unlock()

    println(i,"write over")
}

1 write start

1 writing

2 read start

3 write start

1 writing over

2 reading

2 read over

3 writing

3 write over

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