Convert **T to *unsafe.Pointer in Go

How do I convert a variable of type **T to *unsafe.Pointer?

The example below will give the compilation error:
> cannot convert &ptr (type **s) to type *unsafe.Pointer

package main
import (
	"sync/atomic"
	"unsafe"
)
type s struct {
	value int
}
func main(){
	var ptr *s
	a := &s{42}
	
	old := ptr
	atomic.CompareAndSwapPointer(
		(*unsafe.Pointer)(&ptr), // &unsafe.Pointer(ptr)
		unsafe.Pointer(old),
		unsafe.Pointer(a))
}

If I switch (*unsafe.Pointer)(&ptr) to &unsafe.Pointer(ptr), I will get this compilation error:
> cannot take the address of unsafe.Pointer(ptr)

Ps. I choose to make an example with sync/atomic because that is one situation where you actually have to do such a conversion.

Edit

One incorrect solution would be to use a temporary variable:

up := unsafe.Pointer(ptr)
atomic.CompareAndSwapPointer(&up, ...

While it compiles, the CAS will only swap what is stored in up and not in ptr. This is not the desired result, as zeebo@#go-nuts pointed out.

mcef@#go-nuts posted the answer how to convert **T:
> (*unsafe.Pointer)(unsafe.Pointer(ptr)), where ptr is of type **T.

zeebo@#go-nuts provided a working example (posted here with permission):

package main
import (
	"fmt"
	"sync/atomic"
	"unsafe"
)
type T struct {
	value int
}
func Swap(dest **T, old, new *T) bool {
	udest := (*unsafe.Pointer)(unsafe.Pointer(dest))
	return atomic.CompareAndSwapPointer(udest,
		unsafe.Pointer(old),
		unsafe.Pointer(new),
	)
}
func main() {
	x := &T{42}
	n := &T{50}
	fmt.Println(*x, *n)
	p := x
	Swap(&x, p, n)
	fmt.Println(*x, *n)
}