shared attrribute

[Jack Applegame]

What does mean attribute shared exactly for non global variables? 
Is it safe to cast to/from shared?

For example, let us assume all data are synchronized properly. Is 
following code safe?

import core.atomic;

class A {
  int value;
}

struct B {
  A node;
}

void main() {
  shared B data;
  A node = new A;
  atomicStore(data, shared B(cast(shared) node));
  ...
  node = atomicLoad(data).node;
}