Why does this code cause data race?

Question

1 package main
2
3 import \"time\"
4
5 func main() {
6     m1 := make(map[string]int)
7     m1[\"hello\"] = 1
8     m1[\"world\"] = 2
9     go func() {
10         for i          


        

Answer 1

Data race

A data race occurs when two goroutines access the same variable concurrently and at least one of the accesses is a write.

Instructions Reorder

compilers and processors may reorder the reads and writes executed within a single goroutine as far as the reordering does not change the behaviour within the routine, it doesn' ensure behaviour of other goroutines to be unaffected

m2["hello"] = 3
m1 = m2

Can be reordered to

m1 = m2
m2["hello"] = 3

That won't alter the behaviour of the main routine and thus race checking will consider that also for evaluating race condition. Now we have m2["hello"] = 3 causing the race condition and it prints out the same with its original line number