[英]Golang data race even with mutex for custom concurrent maps
這是我為學習目的編寫的簡單並發映射
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()
}
我正在嘗試編寫一個簡單的基准,我發現同步訪問將正確運行,但並發訪問將獲得
fatal error: concurrent map writes
這是我使用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
是在說bucket.hashMap[key] = val
引起了問題,但是我對為什么可能這樣感到困惑,因為無論何時寫操作都會鎖定該邏輯。
我認為我缺少一些基本知識,有人可以指出我的錯誤嗎?
謝謝
EDIT1:
不知道這是否有幫助,但是如果我不鎖定任何東西,這就是我的互斥鎖的樣子
{{0 0} 0 0 0 0}
這是我鎖定寫入后的樣子
{{1 0} 0 0 -1073741824 0}
不知道為什么我的readerCount是一個低負數
編輯:2
我想我找到了問題所在,但是不確定為什么我必須這樣編碼
問題是
type ThreadSafeMap struct {
mapLock sync.RWMutex // This is causing problem
hashMap map[string]interface{}
}
它應該是
type ThreadSafeMap struct {
mapLock *sync.RWMutex
hashMap map[string]interface{}
}
另一個奇怪的事情是在Put
如果我將print語句放在鎖中
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()
可以進行以下打印
start
start
{0x4212861c0 map[123:123]}
{0x4212241c0 map[123:123]}
這很奇怪,因為每個start
打印輸出后都應跟隨4行存儲桶信息,因為您無法連續背對背start
,因為這將表明多個線程正在訪問鎖內的行
同樣由於某種原因,即使我將bucketIndex設為靜態,每個bucket.mapLock
都有不同的地址,這表明我什至沒有訪問相同的鎖。
但是盡管有上述怪異,但將互斥體更改為指針仍解決了我的問題
我很想找出為什么我需要互斥鎖的指針,以及為什么打印結果似乎表明多個線程正在訪問該鎖以及每個鎖具有不同的地址。
問題在於語句
bucket := cMap.buckets[bucketIndex]
bucket
現在在該索引處包含ThreadSafeMap
副本。 由於sync.RWMutex
作為值存儲,因此在分配時將其復制。 但是,映射映射保留對基礎數據結構的引用,因此將傳遞指針或同一映射的副本。 該代碼在寫入單個映射時會鎖定該鎖的副本,這會導致問題。
因此,將sync.RWMutex
更改為*sync.RWMutex
時,您不會遇到任何問題。 如圖所示,最好將對結構的引用存儲在地圖中。
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()
}
可以通過如下修改Put
函數來驗證場景
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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