简述
Golang中的锁机制主要包含互斥锁和 读写锁
互斥锁
互斥锁是传统并发程序对共享资源进行控制访问的主要手段。在Go中主要使用 sync.Mutex的结构体表示。
一个简单的示例：
func mutex() {
var mu sync.Mutex
mu.Lock()
fmt.Println("locked")
mu.Unlock()
}
或者也可以使用defer来实现，这在整个函数流程中全部要加锁时特别有用，还有一个好处就是可以防止忘记Unlock
func mutex() {
var mu sync.Mutex
mu.Lock()
defer mu.Unlock()
fmt.Println("locked")
}
互斥锁是开箱即用的，只需要申明sync.Mutex即可直接使用
var mu sync.Mutex
互斥锁应该是成对出现，在同步语句不可以再对锁加锁，看下面的示例：
func mutex() {
var mu sync.Mutex
mu.Lock()
fmt.Println("parent locked")
mu.Lock()
fmt.Println("sub locked")
mu.Unlock()
mu.Unlock()
}
此时则会出现fatal error: all goroutines are asleep – deadlock!错误
同样，如果多次对一个锁解锁，则会出现fatal error: sync: unlock of unlocked mutex错误
func mutex() {
var mu sync.Mutex
mu.Lock()
fmt.Println("locked")
mu.Unlock()
mu.Unlock()
}
那么在goroutine中是否对外部锁加锁呢？
func mutex() {
var mu sync.Mutex
fmt.Println("parent lock start")
mu.Lock()
fmt.Println("parent locked")
for i := 0; i <= 2; i++ {
go func(i int) {
fmt. Printf ("sub(%d) lock startn", i)
mu.Lock()
fmt.Printf("sub(%d) lockedn", i)
time.Sleep(time.Microsecond * 30)
mu.Unlock()
fmt.Printf("sub(%d) unlockn", i)
}(i)
}
time.Sleep(time.Second * 2)
mu.Unlock()
fmt.Println("parent unlock")
time.Sleep(time.Second * 2)
}
先看上面的函数执行结果
parent lock start parent locked sub(0) lock start sub(2) lock start sub(1) lock start parent unlock // 必须等到父级先解锁，后面则会阻塞 sub(0) locked // 解锁后子goroutine才能执行锁定 sub(0) unlock sub(2) locked sub(2) unlock sub(1) locked sub(1) unlock
为了方便调试，使用了time.Sleep()来延迟保证goroutine的执行 从结果中可以看出，当所有的goroutine遇到Lock时都会阻塞，而当main函数中的Unlock执行后，会有一个优先（无序）的goroutine来占得锁，其它的则再次进入阻塞状态。
总结：
- 互斥锁必须成对出现
- 同级别互斥锁不能嵌套使用
- 父级中如果存在锁，当在goroutine中执行重复锁定操作时goroutine将被阻塞，直到原互斥锁解锁，多个goroutine将会争抢当前锁资源，其它继续阻塞。
读写锁
读写锁和互斥锁不同之处在于，可以分别针对读操作和写操作进行分别锁定，这样对于性能有一定的提升。 读写锁，对于多个写操作，以及写操作和读操作之前都是互斥的这一点基本等同于互斥锁。 但是对于同时多个读操作之前却非互斥关系，这也是相读写锁性能高于互斥锁的主要原因。
读写锁也是开箱即用型的
var rwm = sync.RWMutex
读写锁分为写锁和读锁：
- 写锁定和写解锁
rwm.Lock() rwm.Unlock()
- 读锁定和读解锁
rwm.RLock() rwm.RUnlock()
读写锁的读锁和写锁不能交叉相互解锁，否则会发生panic，如：
func rwMutex() {
var rwm sync.RWMutex
rwm.Lock()
fmt.Println("locked")
rwm.RUnlock()
}
fatal error: sync: RUnlock of unlocked RWMutex
对于读写锁，同一资源可以同时有多个读锁定，如：
func rwMutex() {
var rwm sync.RWMutex
rwm.RLock()
rwm.RLock()
rwm.RLock()
fmt.Println("locked")
rwm.RUnlock()
rwm.RUnlock()
rwm.RUnlock()
}
但对于写锁定只能有一个（和互斥锁相同），同时使用多个会产生deadlock的panic，如：
func rwMutex() {
var rwm sync.RWMutex
rwm.Lock()
rwm.Lock()
rwm.Lock()
fmt.Println("locked")
rwm.Unlock()
rwm.Unlock()
rwm.Unlock()
}
在goroutine中，写解锁会试图唤醒所有想要进行读锁定而被阻塞的goroutine。
而读解锁会在已无任何读锁定的情况下，试图唤醒一个想进行写锁定而被阻塞的goroutine。
下面看一个完整示例：
func rwMutex() {
var rwm sync.RWMutex
for i := 0; i <= 2; i++ {
go func(i int) {
fmt.Printf("go(%d) start lockn", i)
rwm.RLock()
fmt.Printf("go(%d) lockedn", i)
time.Sleep(time.Second * 2)
rwm.RUnlock()
fmt.Printf("go(%d) unlockn", i)
}(i)
}
// 先sleep一小会，保证for的goroutine都会执行
time.Sleep(time.Microsecond * 100)
fmt.Println("main start lock")
// 当子进程都执行时，且子进程所有的资源都已经Unlock了
// 父进程才会执行
rwm.Lock()
fmt.Println("main locked")
time.Sleep(time.Second)
rwm.Unlock()
}
go(0) start lock go(0) locked go(1) start lock go(1) locked go(2) start lock go(2) locked main start lock go(2) unlock go(0) unlock go(1) unlock main locked
反复执行上述示例中，可以看到，写锁定会阻塞goroutine 最开始先在main中sleep 100ms ，保证子的goroutine会全部执行，而每个子goroutine会sleep 2s。 此时会阻塞整个main进程，当所有子goroutine执行结束，读解锁后，main的写锁定才会执行。
再看一个读锁定示例：
func rwMutex5() {
var rwm sync.RWMutex
for i := 0; i <= 2; i++ {
go func(i int) {
fmt.Printf("go(%d) start lockn", i)
rwm.RLock()
fmt.Printf("go(%d) lockedn", i)
time.Sleep(time.Second * 2)
rwm.RUnlock()
fmt.Printf("go(%d) unlockn", i)
}(i)
}
fmt.Println("main start lock")
rwm.RLock()
fmt.Println("main locked")
time.Sleep(time.Second * 10)
}
main start lock main locked go(1) start lock go(1) locked go(2) start lock go(2) locked go(0) start lock go(0) locked go(0) unlock go(1) unlock go(2) unlock
可以看到读锁定却并不会阻塞goroutine。
总结：
- 读锁定和写锁定对于写操作都是互斥的
- 读锁定支持多级嵌套，但写锁定无法嵌套执行
- 如果有写锁定，当多个读解锁全部执行完成后，则会唤起执行写锁定
- 写锁定会阻塞goroutine（在Lock()时和互斥锁一样，RLock()时先也是等到RUnlock()先执行，才有锁定机会）
感谢阅读
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