Mutex within loop leads to unexpected output

I have this simplistic piece of code (or here https://play.golang.org/p/KW8_OHUp9v)

package main

import (
	"fmt"
	"sync"
)

func main() {
	mutex := new(sync.Mutex)

	for i := 1; i < 5; i++ {
		for j := 1; j < 5; j++ {
			mutex.Lock()
			go func() {
				fmt.Printf("%d + %d = %d\n", i, j, j+i)
				mutex.Unlock()
			}()
		}
	}
}

It produces an output like this

1 + 2 = 3
1 + 3 = 4
1 + 4 = 5
2 + 5 = 7
2 + 2 = 4
2 + 3 = 5
2 + 4 = 6
3 + 5 = 8
3 + 2 = 5
3 + 3 = 6
3 + 4 = 7
4 + 5 = 9
4 + 2 = 6
4 + 3 = 7
4 + 4 = 8

Program exited.

Looking at the output I was surprised by few things:

1. There are no '1's for the j
2. There are '5's for the j
3. There are only 3 values for i=1, instead of 4

I can understand lack of '1's as the variable is incremented before it is written.

Can someone explain 2. and 3. ?

You're closing over variables in a loop and then running the closure in a separate thread, while those variables continue to change. When you do this, expect the unexpected - for example, you see 5s because j is incremented to 5 on the last iteration, which causes the loop to end, but j still holds 5, which the separate thread can then read. It has nothing to do with your mutex; it's the cross-thread sharing of variables. If you use:

go func(i,j int) {
    fmt.Printf("%d + %d = %d\n", i, j, j+i)
    mutex.Unlock()
}(i,j)

Then it will pass in the values of i and j at the time your goroutine is started, and subsequent iterations won't affect it: https://play.golang.org/p/P3kUP5e1Fp

When you execute this :

go func() {
                fmt.Printf("%d + %d = %d\n", i, j, j+i)
                mutex.Unlock()
            }()

The current goroutine makes another loop which increments j.

Incrementation takes place before printf that is why even thought the function was called while
j was < 5 it could be increased to 5 before the function had time to print out values.
In other words you program runs like this :

• Enter loop
• Lock Mutex
• Call func()
• Increment j
• Print values
• Unlock Mutex

Solution to this would be to pass the values by value to the function instead of sharing them throughout goroutines.