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 (
"time"
"fmt"
"sync"
)
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: 1
Not lock: 2
Not lock: 0
Not lock: 3
Unlock the lock
Locked: 1
Unlock the lock: 1
Locked: 2
Unlock the lock: 2
Locked: 3
Unlock the lock: 3
Locked: 0
Unlock the lock: 0
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 (
"sync"
"fmt"
"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 the lock
The lock is locked
Not lock: 0
Not lock: 1
Not lock: 2
Not lock: 3
Unlock the lock
Locked: 0
Unlock the lock: 0
Locked: 2
Unlock the lock: 2
Locked: 3
Unlock the lock: 3
Locked: 1
Unlock the lock: 1
Lock() 和 RLock()
package main
import (
"sync"
"fmt"
"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)
fmt.Println("Unlock the read lock: ", i)
time.Sleep(time.Second)
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: 2
Not read lock: 3
Not read lock: 1
Not read lock: 0
Unlock the lock
Read Locked: 2
Read Locked: 1
Unlock the read lock: 2
Unlock the read lock: 1
Read Locked: 0
Read Locked: 3
Unlock the read lock: 0
Unlock the read lock: 3
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：
panic: 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：
panic: 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：
panic: sync: RUnlock of unlocked RWMutex