sync.Map seems not safe concurrent read/write - go

Question

Test the sync.Map in golang standard package. It seems not safe concurrent read and write.
What's wrong?
Test code:
package main
import (
"log"
"sync"
)
func main() {
var m sync.Map
m.Store("count", 0)
var wg sync.WaitGroup
for numOfThread := 0; numOfThread < 10; numOfThread++ {
wg.Add(1)
go func() {
defer wg.Done()
for i := 0; i < 1000; i++ {
value, ok := m.Load("count")
if !ok {
log.Println("load count error")
} else {
v, _ := value.(int)
m.Store("count", v+1)
}
}
}()
}
log.Println("threads starts")
wg.Wait()
value, ok := m.Load("count")
if ok {
v, _ := value.(int)
log.Printf("final count: %d", v)
}
log.Println("all done")
}
https://play.golang.org/p/E-pw4iZUceB
The result should be 10000, but get random number but not 10000:
2009/11/10 23:00:00 threads starts
2009/11/10 23:00:00 final count: 6696
2009/11/10 23:00:00 all done

Answer 1

You have a race condition:
value, ok := m.Load("count")
...
v, _ := value.(int)
m.Store("count", v+1)
The read-modify-store above does not protect other goroutines do the same thing, thus some of the increments performed by other goroutines will be missed.
The sync.Map protects concurrent access to its members. That means, a write to the map will not cause other goroutines to read an inconsistent map. If you read-modify-write, nothing will protect other goroutines from updating the value at the same time. You need a mutex to protect access to the map when you read-modify-update.

Answer 2

package main
import (
"log"
"sync"
)
func main() {
var m sync.Map
m.Store("count", 0)
var wg sync.WaitGroup
var mu *sync.Mutex
for numOfThread := 0; numOfThread < 10; numOfThread++ {
wg.Add(1)
go func() {
defer wg.Done()
for i := 0; i < 1000; i++ {
mu.Lock()
value, ok := m.Load("count")
if !ok {
log.Println("load count error")
} else {
v, _ := value.(int)
m.Store("count", v+1)
}
mu.Unlock()
}
}()
}
log.Println("threads starts")
wg.Wait()
value, ok := m.Load("count")
if ok {
v, _ := value.(int)
log.Printf("final count: %d", v)
}
log.Println("all done")
}
both read and write operation are thread safe, but you are attempting an upsert or read+write operation. It is not thread safe. Modified code to make it thread safe.