[英]Golang data race even with mutex for custom concurrent maps
这是我为学习目的编写的简单并发映射
package concurrent_hashmap
import (
"hash/fnv"
"sync"
)
type ConcurrentMap struct {
buckets []ThreadSafeMap
bucketCount uint32
}
type ThreadSafeMap struct {
mapLock sync.RWMutex
hashMap map[string]interface{}
}
func NewConcurrentMap(bucketSize uint32) *ConcurrentMap {
var threadSafeMapInstance ThreadSafeMap
var bucketOfThreadSafeMap []ThreadSafeMap
for i := 0; i <= int(bucketSize); i++ {
threadSafeMapInstance = ThreadSafeMap{sync.RWMutex{}, make(map[string]interface{})}
bucketOfThreadSafeMap = append(bucketOfThreadSafeMap, threadSafeMapInstance)
}
return &ConcurrentMap{bucketOfThreadSafeMap, bucketSize}
}
func (cMap *ConcurrentMap) Put(key string, val interface{}) {
bucketIndex := hash(key) % cMap.bucketCount
bucket := cMap.buckets[bucketIndex]
bucket.mapLock.Lock()
bucket.hashMap[key] = val
bucket.mapLock.Unlock()
}
// Helper
func hash(s string) uint32 {
h := fnv.New32a()
h.Write([]byte(s))
return h.Sum32()
}
我正在尝试编写一个简单的基准,我发现同步访问将正确运行,但并发访问将获得
fatal error: concurrent map writes
这是我使用go test -bench=. -race
基准go test -bench=. -race
go test -bench=. -race
package concurrent_hashmap
import (
"testing"
"runtime"
"math/rand"
"strconv"
"sync"
)
// Concurrent does not work
func BenchmarkMyFunc(b *testing.B) {
var wg sync.WaitGroup
runtime.GOMAXPROCS(runtime.NumCPU())
my_map := NewConcurrentMap(uint32(4))
for n := 0; n < b.N; n++ {
go insert(my_map, wg)
}
wg.Wait()
}
func insert(my_map *ConcurrentMap, wg sync.WaitGroup) {
wg.Add(1)
var rand_int int
for element_num := 0; element_num < 1000; element_num++ {
rand_int = rand.Intn(100)
my_map.Put(strconv.Itoa(rand_int), rand_int)
}
defer wg.Done()
}
// This works
func BenchmarkMyFuncSynchronize(b *testing.B) {
my_map := NewConcurrentMap(uint32(4))
for n := 0; n < b.N; n++ {
my_map.Put(strconv.Itoa(123), 123)
}
}
WARNING: DATA RACE
bucket.hashMap[key] = val
是在说bucket.hashMap[key] = val
引起了问题,但是我对为什么可能这样感到困惑,因为无论何时写操作都会锁定该逻辑。
我认为我缺少一些基本知识,有人可以指出我的错误吗?
谢谢
EDIT1:
不知道这是否有帮助,但是如果我不锁定任何东西,这就是我的互斥锁的样子
{{0 0} 0 0 0 0}
这是我锁定写入后的样子
{{1 0} 0 0 -1073741824 0}
不知道为什么我的readerCount是一个低负数
编辑:2
我想我找到了问题所在,但是不确定为什么我必须这样编码
问题是
type ThreadSafeMap struct {
mapLock sync.RWMutex // This is causing problem
hashMap map[string]interface{}
}
它应该是
type ThreadSafeMap struct {
mapLock *sync.RWMutex
hashMap map[string]interface{}
}
另一个奇怪的事情是在Put
如果我将print语句放在锁中
bucket.mapLock.Lock()
fmt.Println("start")
fmt.Println(bucket)
fmt.Println(bucketIndex)
fmt.Println(bucket.mapLock)
fmt.Println(&bucket.mapLock)
bucket.hashMap[key] = val
defer bucket.mapLock.Unlock()
可以进行以下打印
start
start
{0x4212861c0 map[123:123]}
{0x4212241c0 map[123:123]}
这很奇怪,因为每个start
打印输出后都应跟随4行存储桶信息,因为您无法连续背对背start
,因为这将表明多个线程正在访问锁内的行
同样由于某种原因,即使我将bucketIndex设为静态,每个bucket.mapLock
都有不同的地址,这表明我什至没有访问相同的锁。
但是尽管有上述怪异,但将互斥体更改为指针仍解决了我的问题
我很想找出为什么我需要互斥锁的指针,以及为什么打印结果似乎表明多个线程正在访问该锁以及每个锁具有不同的地址。
问题在于语句
bucket := cMap.buckets[bucketIndex]
bucket
现在在该索引处包含ThreadSafeMap
副本。 由于sync.RWMutex
作为值存储,因此在分配时将其复制。 但是,映射映射保留对基础数据结构的引用,因此将传递指针或同一映射的副本。 该代码在写入单个映射时会锁定该锁的副本,这会导致问题。
因此,将sync.RWMutex
更改为*sync.RWMutex
时,您不会遇到任何问题。 如图所示,最好将对结构的引用存储在地图中。
package concurrent_hashmap
import (
"hash/fnv"
"sync"
)
type ConcurrentMap struct {
buckets []*ThreadSafeMap
bucketCount uint32
}
type ThreadSafeMap struct {
mapLock sync.RWMutex
hashMap map[string]interface{}
}
func NewConcurrentMap(bucketSize uint32) *ConcurrentMap {
var threadSafeMapInstance *ThreadSafeMap
var bucketOfThreadSafeMap []*ThreadSafeMap
for i := 0; i <= int(bucketSize); i++ {
threadSafeMapInstance = &ThreadSafeMap{sync.RWMutex{}, make(map[string]interface{})}
bucketOfThreadSafeMap = append(bucketOfThreadSafeMap, threadSafeMapInstance)
}
return &ConcurrentMap{bucketOfThreadSafeMap, bucketSize}
}
func (cMap *ConcurrentMap) Put(key string, val interface{}) {
bucketIndex := hash(key) % cMap.bucketCount
bucket := cMap.buckets[bucketIndex]
bucket.mapLock.Lock()
bucket.hashMap[key] = val
bucket.mapLock.Unlock()
}
// Helper
func hash(s string) uint32 {
h := fnv.New32a()
h.Write([]byte(s))
return h.Sum32()
}
可以通过如下修改Put
函数来验证场景
func (cMap *ConcurrentMap) Put(key string, val interface{}) {
//fmt.Println("index", key)
bucketIndex := 1
bucket := cMap.buckets[bucketIndex]
fmt.Printf("%p %p\n", &(bucket.mapLock), bucket.hashMap)
}
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