在Go1.5中执行GC后，此代码会引起恐慌吗？

package main

import "time"

var x = []string{}

func main() {
    go func() {
        for {
            y := x
            y = append(y, "aa")
        }
    }()
    go func() {
        for {
           x = []string{"123"}
        }
    }()
    for {
        time.Sleep(1)
    }
}

guess like when x (for example, the address of 123) is not really assigned to y, and x is assigned to a new address like 124. and just this time gc happened, would the address of 123 be recycled and cause a panic?

No. First of all, x has a global scope. So GC won't happen until a new value with new address is assign to it.

Now when a new value is assign, two things can happen:

1. Goroutine assigning y to x happens. It get assign to y. Then no GC will happen.
2. It will get new value with new address before first step happens.

I don't know what you are trying to do. But there's no panic even if goroutine runs simultaneously. X always has a value.

Since you asked that whether assignment is atomic: No normal assignment is atomic.

"is not really assigned" - what, there is no such thing in go.

Your code has a race condition go run -race, it writes to "x", reads from "x" and grows "x"'s slice at the same time.

"y := x" is not atomic in general, it depends on value, but it is not atomic for slice/interface{}/map/struct for sure. There is sync.atomic package for atomic primitives.