go中的sync.Mutex 和 sync.RWMutex

左手的ㄟ右手 2022-12-24 15:50 199阅读 0赞

## 介绍 ##

golang 中的 sync 包实现了两种锁：

*  Mutex：互斥锁
 *  RWMutex：读写锁，RWMutex 基于 Mutex 实现

## Mutex（互斥锁） ##

*  Mutex 为互斥锁，Lock() 加锁，Unlock() 解锁
 *  在一个 goroutine 获得 Mutex 后，其他 goroutine 只能等到这个 goroutine 释放该 Mutex
 *  使用 Lock() 加锁后，不能再继续对其加锁，直到利用 Unlock() 解锁后才能再加锁
 *  在 Lock() 之前使用 Unlock() 会导致 panic 异常
 *  已经锁定的 Mutex 并不与特定的 goroutine 相关联，这样可以利用一个 goroutine 对其加锁，再利用其他 goroutine 对其解锁
 *  在同一个 goroutine 中的 Mutex 解锁之前再次进行加锁，会导致死锁
 *  适用于读写不确定，并且只有一个读或者写的场景

### 示例 ###

加锁和解锁示例

package main
    
    import (
       "fmt"
       "sync"
       "time"
    )
    
    func main() { 
       var mutex sync.Mutex
       fmt.Println("Lock the lock")
       mutex.Lock()
       fmt.Println("The lock is locked")
       channels := make([]chan int, 4)
       for i := 0; i < 4; i++ { 
          channels[i] = make(chan int)
          go func(i int, c chan int) { 
             fmt.Println("Not lock: ", i)
             mutex.Lock()
             fmt.Println("Locked: ", i)
             time.Sleep(time.Second)
             fmt.Println("Unlock the lock: ", i)
             mutex.Unlock()
             c <- i
          }(i, channels[i])
       }
       time.Sleep(time.Second)
       fmt.Println("Unlock the lock")
       mutex.Unlock()
       time.Sleep(time.Second)
    
       for _, c := range channels { 
          <-c
       }
    }

程序输出：

Lock the lock
    The lock is locked
    Not lock:  0
    Not lock:  3
    Not lock:  1
    Not lock:  2
    Unlock the lock
    Locked:  0
    Unlock the lock:  0
    Locked:  3
    Unlock the lock:  3
    Locked:  1
    Unlock the lock:  1
    Locked:  2
    Unlock the lock:  2

在解锁之前加锁会导致死锁

package main
    
    import (
       "fmt"
       "sync"
    )
    
    func main() { 
       var mutex sync.Mutex
       mutex.Lock()
       fmt.Println("Locked")
       mutex.Lock()
    }

程序输出

Locked
    fatal error: all goroutines are asleep - deadlock!

## RWMutex（读写锁） ##

*  RWMutex 是单写多读锁，该锁可以加多个读锁或者一个写锁
 *  读锁占用的情况下会阻止写，不会阻止读，多个 goroutine 可以同时获取读锁
 *  写锁会阻止其他 goroutine（无论读和写）进来，整个锁由该 goroutine 独占
 *  适用于读多写少的场景

### Lock() 和 Unlock() ###

*  Lock() 加写锁，Unlock() 解写锁
 *  如果在加写锁之前已经有其他的读锁和写锁，则 Lock() 会阻塞直到该锁可用，为确保该锁可用，已经阻塞的 Lock() 调用会从获得的锁中排除新的读取器，即写锁权限高于读锁，有写锁时优先进行写锁定
 *  在 Lock() 之前使用 Unlock() 会导致 panic 异常

### RLock() 和 RUnlock() ###

*  RLock() 加读锁，RUnlock() 解读锁
 *  RLock() 加读锁时，如果存在写锁，则无法加读锁；当只有读锁或者没有锁时，可以加读锁，读锁可以加载多个
 *  RUnlock() 解读锁，RUnlock() 撤销单词 RLock() 调用，对于其他同时存在的读锁则没有效果
 *  在没有读锁的情况下调用 RUnlock() 会导致 panic 错误
 *  RUnlock() 的个数不得多余 RLock()，否则会导致 panic 错误

### 示例 ###

Lock() 和 Unlock()

package main
    
    import (
       "fmt"
       "sync"
       "time"
    )
    
    func main() { 
       var mutex *sync.RWMutex
       mutex = new(sync.RWMutex)
       fmt.Println("Lock the lock")
       mutex.Lock()
       fmt.Println("The lock is locked")
    
       channels := make([]chan int, 4)
       for i := 0; i < 4; i++ { 
          channels[i] = make(chan int)
          go func(i int, c chan int) { 
             fmt.Println("Not lock: ", i)
             mutex.Lock()
             fmt.Println("Locked: ", i)
             fmt.Println("Unlock the lock: ", i)
             mutex.Unlock()
             c <- i
          }(i, channels[i])
       }
       time.Sleep(time.Second)
       fmt.Println("Unlock the lock")
       mutex.Unlock()
       time.Sleep(time.Second)
    
       for _, c := range channels { 
          <-c
       }
    }

程序输出：

Lock() 和 RLock()

package main
    
    import (
       "fmt"
       "sync"
       "time"
    )
    
    func main() { 
       var mutex *sync.RWMutex
       mutex = new(sync.RWMutex)
       fmt.Println("Lock the lock")
       mutex.Lock()
       fmt.Println("The lock is locked")
    
       channels := make([]chan int, 4)
       for i := 0; i < 4; i++ { 
          channels[i] = make(chan int)
          go func(i int, c chan int) { 
             fmt.Println("Not read lock: ", i)
             mutex.RLock()
             fmt.Println("Read Locked: ", i)
    
             time.Sleep(time.Second)
             fmt.Println("Unlock the read lock: ", i)
             mutex.RUnlock()
             c <- i
          }(i, channels[i])
       }
       time.Sleep(time.Second)
       fmt.Println("Unlock the lock")
       mutex.Unlock()
       time.Sleep(time.Second)
    
       for _, c := range channels { 
          <-c
       }
    }

程序输出：

Lock the lock
    The lock is locked
    Not read lock:  0
    Not read lock:  3
    Not read lock:  1
    Not read lock:  2
    Unlock the lock
    Read Locked:  2
    Read Locked:  0
    Read Locked:  3
    Read Locked:  1
    Unlock the read lock:  1
    Unlock the read lock:  3
    Unlock the read lock:  2
    Unlock the read lock:  0

Unlock() 使用之前不存在 Lock()

package main
    
    import (
       "sync"
    )
    
    func main() { 
       var rwmutex *sync.RWMutex
       rwmutex = new(sync.RWMutex)
       rwmutex.Unlock()
    }

程序输出：

fatal error: sync: Unlock of unlocked RWMutex

RWMutex 使用不当导致的死锁  
示例1：

package main
    
    import (
       "sync"
    )
    
    func main() { 
       var rwmutex *sync.RWMutex
       rwmutex = new(sync.RWMutex)
       rwmutex.Lock()
       rwmutex.Lock()
    }

程序输出：

fatal error: all goroutines are asleep - deadlock!

示例2：

package main
    
    import (
       "sync"
    )
    
    func main() { 
       var rwmutex *sync.RWMutex
       rwmutex = new(sync.RWMutex)
       rwmutex.Lock()
       rwmutex.RLock()
    }

程序输出：

fatal error: all goroutines are asleep - deadlock!

RUnlock() 之前不存在 RLock()

package main
    
    import (
       "sync"
    )
    
    func main() { 
       var rwmutex *sync.RWMutex
       rwmutex = new(sync.RWMutex)
       rwmutex.RUnlock()
    }

程序输出：

fatal error: sync: RUnlock of unlocked RWMutex

RUnlock() 个数多于 RLock()

package main
    
    import (
       "sync"
    )
    
    func main() { 
       var rwmutex *sync.RWMutex
       rwmutex = new(sync.RWMutex)
       rwmutex.RLock()
       rwmutex.RLock()
       rwmutex.RUnlock()
       rwmutex.RUnlock()
       rwmutex.RUnlock()
    }

程序输出：

fatal error: sync: RUnlock of unlocked RWMutex

转自：https://andblog.cn/?p=2815