英文:
Is this a right way to update a global golang map with concurrent read?
问题
这是一个致命错误：并发地读取和写入映射的演示代码：
package main
var Map = make(map[int]int)
func main() {
for i := 0; i < 100000; i++ {
go updateMap()
go readMap(i)
}
}
func readMap(key int) int {
return Map[key]
}
func updateMap() {
for j := 0; j < 1000; j++ {
Map[j] = j
}
}
我没有使用锁或sync.Map，只是使用临时变量替换全局映射，这样就不会发生panic，代码如下：
package main
var Map = make(map[int]int)
func main() {
for i := 0; i < 100000; i++ {
go updateMap()
go readMap(i)
}
}
func readMap(key int) int {
return Map[key]
}
func updateMap() {
tmp := map[int]int{}
for j := 0; j < 1000; j++ {
tmp[j] = j
}
Map = tmp
}
这种更新全局映射的方式有问题吗？
英文:
this is a fatal error: concurrent map read and map write demo code:
package main
var Map = make(map[int]int)
func main() {
for i := 0; i < 100000; i++ {
go updateMap()
go readMap(i)
}
}
func readMap(key int) int {
return Map[key]
}
func updateMap() {
for j := 0; j < 1000; j++ {
Map[j] = j
}
}
I don't use the lock or sync.Map, just use tmp var to replace global map, it will not panic, codes like this:
package main
var Map = make(map[int]int)
func main() {
for i := 0; i < 100000; i++ {
go updateMap()
go readMap(i)
}
}
func readMap(key int) int {
return Map[key]
}
func updateMap() {
tmp := map[int]int{}
for j := 0; j < 1000; j++ {
tmp[j] = j
}
Map = tmp
}
Is there any problem with this way to update the global map?
答案1
得分: 1
更新地图的第二种方式不会引发恐慌，因为被更新的地图是一个临时地图，但仍然存在竞争条件。在updateMap函数之后，无法保证其他goroutine何时会看到updateMap的效果，如果有的话。
正确的方式是对两种情况都使用RWMutex。readMap应该使用RLock/RUnlock，而updateMap应该使用Lock/Unlock。对于第二种情况，如果updateMap是唯一更新地图的goroutine，你可以在复制阶段使用Rlock/RUnlock，在分配地图时使用Lock/Unlock。
英文:
The second way of updating the map will not panic, because the map being updated is a temporary map, but there is still a race condition. After the updateMap function, there is no guarantee when the other goroutine will see the effects of updateMap, if ever.
The correct way is to use a RWMutex for both cases. The readMap should use RLock/RUnlock, and updateMap should use Lock/Unlock. For the second case, if the updateMap the only goroutine that ever updates the map, you can use Rlock/RUnlock during the copy phase, and Lock/Unlock when you assign the map.
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