使用-race标志时，GO中的竞争条件意味着什么

Question

Consider following code:

package main

import (
    "fmt"
    "sync"
)

func main() {
var a int
m := new(sync.Mutex)

wg := sync.WaitGroup{}
wg.Add(2)

go func(){
    m.Lock()
    a = 2
    m.Unlock()
    wg.Done()
}()

go func(){
    //m.Lock()
    a = 9
    //m.Unlock()
    wg.Done()
}()

wg.Wait()

fmt.Println(a)
}

if we run this code with -race flag we get warning that there is a race condition.

1) what can possibly go wrong with this race condition?

if we uncomment lock in second routine we don't get a race condition warning. But we can have different output so a race condition exists.

2) Why now we don't have a race condition warning?

Answer 1

1) what can possibly go wrong with this race condition?

• Undefined behavior
• memory corruption
• crashes Literally the worst possible outcome of not knowing and potentially having a completely invalid value in your variable :p

2) Why now we don't have a race condition warning?

Mutex is a primitive which can guarantee atomicity. When lock is uncommented the runtime/os fully syncrhonizes access to the statement(s) that the lock protects. ie a will never be set to both 2 and 9 at the same time.

There may be a performance implications here (your app may never run into them) because these are serialized operations. This is usually a fine tradeoff because it ensures correctness at the cost of potential performance implications.

---

The go documentation has amazing resources around the specifics of this issue:

• https://golang.org/ref/mem
• https://golang.org/doc/articles/race_detector.html
• https://golang.org/pkg/sync/
• https://golang.org/pkg/sync/atomic/