GO語言協程互斥鎖Mutex和讀寫鎖RWMutex用法範例詳解

// A Mutex is a mutual exclusion lock.
// The zero value for a Mutex is an unlocked mutex.
//
// A Mutex must not be copied after first use.
type Mutex struct {
	state int32
	sema uint32
}

package main

import (
	"fmt"
	"sync"
	"time"
)

// 共用變數
var (
	m  sync.Mutex
	v1 int
)

// 修改共用變數
// 在Lock()和Unlock()之間的程式碼部分是臨界區
func change(i int) {
	m.Lock()
	time.Sleep(time.Second)
	v1 = v1 + 1
	if v1%10 == 0 {
		v1 = v1 - 10*i
	}
	m.Unlock()
}

// 存取共用變數
// 在Lock()和Unlock()之間的程式碼部分是是臨界區
func read() int {
	m.Lock()
	a := v1
	m.Unlock()
	return a
}

func main() {
	var numGR = 21
	var wg sync.WaitGroup

	fmt.Printf("%d", read())

	// 迴圈建立numGR個goroutine
	// 每個goroutine都執行change()、read()
	// 每個change()和read()都會持有鎖
	for i := 0; i < numGR; i++ {
		wg.Add(1)
		go func(i int) {
			defer wg.Done()
			change(i)
			fmt.Printf(" -> %d", read())
		}(i)
	}

	wg.Wait()
}

0 -> 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 -> -100 -> -99
-> -98 -> -97 -> -96 -> -95 -> -94 -> -93 -> -92 -> -91 -> -260 -> -259

0 -> 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 -> -80 -> -79 
-> -78 -> -77 -> -76 -> -75 -> -74 -> -72 -> -71 -> -230 -> -229 -> -229

type mytype struct {
	m   sync.Mutex
	var int
}

x := new(mytype)

// A RWMutex is a reader/writer mutual exclusion lock.
// The lock can be held by an arbitrary number of readers or a single writer.
// The zero value for a RWMutex is an unlocked mutex.
//
// A RWMutex must not be copied after first use.
//
// If a goroutine holds a RWMutex for reading and another goroutine might
// call Lock, no goroutine should expect to be able to acquire a read lock
// until the initial read lock is released. In particular, this prohibits
// recursive read locking. This is to ensure that the lock eventually becomes
// available; a blocked Lock call excludes new readers from acquiring the
// lock.
type RWMutex struct {
	w           Mutex  // held if there are pending writers
	writerSem   uint32 // 寫鎖需要等待讀鎖釋放的號誌
	readerSem   uint32 // 讀鎖需要等待寫鎖釋放的號誌
	readerCount int32  // 讀鎖後面掛起了多少個寫鎖申請
	readerWait  int32  // 已釋放了多少個讀鎖
}

func (rw *RWMutex) Lock()
func (rw *RWMutex) RLock()
func (rw *RWMutex) RLocker() Locker
func (rw *RWMutex) RUnlock()
func (rw *RWMutex) Unlock()

package main

import (
	"fmt"
	"os"
	"sync"
	"time"
)

var Password = secret{password: "myPassword"}

type secret struct {
	RWM      sync.RWMutex
	M        sync.Mutex
	password string
}

// 通過rwmutex寫
func Change(c *secret, pass string) {
	c.RWM.Lock()
	fmt.Println("Change with rwmutex lock")
	time.Sleep(3 * time.Second)
	c.password = pass
	c.RWM.Unlock()
}

// 通過rwmutex讀
func rwMutexShow(c *secret) string {
	c.RWM.RLock()
	fmt.Println("show with rwmutex",time.Now().Second())
	time.Sleep(1 * time.Second)
	defer c.RWM.RUnlock()
	return c.password
}

// 通過mutex讀，和rwMutexShow的唯一區別在於鎖的方式不同
func mutexShow(c *secret) string {
	c.M.Lock()
	fmt.Println("show with mutex:",time.Now().Second())
	time.Sleep(1 * time.Second)
	defer c.M.Unlock()
	return c.password
}

func main() {
	// 定義一個稍後用於覆蓋(重寫)的函數
	var show = func(c *secret) string { return "" }

	// 通過變數賦值的方式，選擇並重寫showFunc函數
	if len(os.Args) != 2 {
		fmt.Println("Using sync.RWMutex!",time.Now().Second())
		show = rwMutexShow
	} else {
		fmt.Println("Using sync.Mutex!",time.Now().Second())
		show = mutexShow
	}
	
	var wg sync.WaitGroup

	// 啟用5個goroutine，每個goroutine都檢視
	// 根據選擇的函數不同，showFunc()加鎖的方式不同
	for i := 0; i < 5; i++ {
		wg.Add(1)
		go func() {
			defer wg.Done()
			fmt.Println("Go Pass:", show(&Password),time.Now().Second())
		}()
	}
	
	// 啟用一個申請寫鎖的goroutine
	go func() {
		wg.Add(1)
		defer wg.Done()
		Change(&Password, "123456")
	}()
	// 阻塞，直到所有wg.Done
	wg.Wait()
}

Using sync.RWMutex! 58
show with rwmutex 58
Change with rwmutex lock
Go Pass: myPassword 59
show with rwmutex 2
show with rwmutex 2
show with rwmutex 2
show with rwmutex 2
Go Pass: 123456 3
Go Pass: 123456 3
Go Pass: 123456 3
Go Pass: 123456 3

Using sync.Mutex! 30
Change with rwmutex lock
show with mutex: 30
Go Pass: myPassword 31
show with mutex: 31
Go Pass: myPassword 32
show with mutex: 32
Go Pass: 123456 33
show with mutex: 33
show with mutex: 34
Go Pass: 123456 34
Go Pass: 123456 35