为什么我的 golang 无锁队列总是卡在那里？

Question

这是我的代码：

package main

import (
    "sync/atomic"
    "unsafe"
    "sync"
    "fmt"
    "time"
)

const (
    MAX_DATA_SIZE = 100
)

// lock free queue
type Queue struct {
    head unsafe.Pointer
    tail unsafe.Pointer
}
// one node in queue
type Node struct {
    val interface{}
    next unsafe.Pointer
}
// queue functions
func (self *Queue) enQueue(val interface{}) {
    newValue := unsafe.Pointer(&Node{val: val, next: nil})
    var tail,next unsafe.Pointer
    for {
        tail = self.tail
        next = ((*Node)(tail)).next
        if next != nil {
            atomic.CompareAndSwapPointer(&(self.tail), tail, next)
        }else if atomic.CompareAndSwapPointer(&((*Node)(tail).next), nil, newValue){
            break
        }
    }
}

func (self *Queue) deQueue() (val interface{}, success bool){
    var head,tail,next unsafe.Pointer
    for {
        head = self.head
        tail = self.tail
        next = ((*Node)(head)).next
        if head == tail {
            if next == nil {
                return nil, false
            }else {
                atomic.CompareAndSwapPointer(&(self.tail), tail, next)
            }
        }else {
            val = ((*Node)(next)).val
            if atomic.CompareAndSwapPointer(&(self.head), head, next) {
                return val, true
            }
        }
    }
    return
}

func main() {
    var wg sync.WaitGroup
    wg.Add(20)
    queue := new(Queue)
    queue.head = unsafe.Pointer(new(Node))
    queue.tail = queue.head

    for i := 0; i < 10; i++ {
        go func() {
            defer wg.Done()
            for j := 0; j < MAX_DATA_SIZE; j++ {
                t := time.Now()
                queue.enQueue(t)
                fmt.Println("enq = ", t)
            }
        }()
    }

    for i := 0; i < 10; i++ {
        go func() {
            ok := false
            var val interface{}
            defer wg.Done()
            for j := 0; j < MAX_DATA_SIZE; j++ {
                val,ok = queue.deQueue()
                for !ok {
                    val,ok = queue.deQueue()
                }
                fmt.Println("deq = ",val)
            }
        }()
    }

    wg.Wait()
}

问题是，有时代码运行正常，但有时它会失败并且卡住没有响应。

我的代码有问题吗？

Answer 1

这段代码中有很多主动等待，我强烈建议像 Nick 的漂亮代码一样干净地使用 channel。

但是，这是我对原始问题“为什么卡住了？”的回答。: 没有保证每个 goroutine 什么时候会让其他人执行，而且很可能在无限循环中永远不会让步。

您可以通过在每个可能无限循环中使用runtime.Gosched()来解决此问题：

Gosched 产生处理器，允许其他 goroutine 运行。它不会挂起当前的 goroutine，因此会自动恢复执行。

此增强代码的运行速度几乎与原始代码一样快，但永远不会挂起：

package main

import (
    "fmt"
    "runtime"
    "sync"
    "sync/atomic"
    "time"
    "unsafe"
)

const (
    MAX_DATA_SIZE = 100
)

// lock free queue
type Queue struct {
    head unsafe.Pointer
    tail unsafe.Pointer
}

// one node in queue
type Node struct {
    val  interface{}
    next unsafe.Pointer
}

// queue functions
func (self *Queue) enQueue(val interface{}) {
    newValue := unsafe.Pointer(&Node{val: val, next: nil})
    var tail, next unsafe.Pointer
    for {
        tail = self.tail
        next = ((*Node)(tail)).next
        if next != nil {
            atomic.CompareAndSwapPointer(&(self.tail), tail, next)
        } else if atomic.CompareAndSwapPointer(&((*Node)(tail).next), nil, newValue) {
            break
        }
        runtime.Gosched()
    }
}

func (self *Queue) deQueue() (val interface{}, success bool) {
    var head, tail, next unsafe.Pointer
    for {
        head = self.head
        tail = self.tail
        next = ((*Node)(head)).next
        if head == tail {
            if next == nil {
                return nil, false
            } else {
                atomic.CompareAndSwapPointer(&(self.tail), tail, next)
            }
        } else {
            val = ((*Node)(next)).val
            if atomic.CompareAndSwapPointer(&(self.head), head, next) {
                return val, true
            }
        }
        runtime.Gosched()
    }
    return
}

func main() {
    var wg sync.WaitGroup
    wg.Add(20)
    queue := new(Queue)
    queue.head = unsafe.Pointer(new(Node))
    queue.tail = queue.head

    for i := 0; i < 10; i++ {
        go func() {
            defer wg.Done()
            for j := 0; j < MAX_DATA_SIZE; j++ {
                t := time.Now()
                queue.enQueue(t)
                fmt.Println("enq = ", t)
            }
        }()
    }

    for i := 0; i < 10; i++ {
        go func() {
            ok := false
            var val interface{}
            defer wg.Done()
            for j := 0; j < MAX_DATA_SIZE; j++ {
                val, ok = queue.deQueue()
                for !ok {
                    val, ok = queue.deQueue()
                    runtime.Gosched()
                }
                fmt.Println("deq = ", val)
            }
        }()
    }

    wg.Wait()
}