单例模式下volatile可见性问题

public class Singleton {
private volatile static Singleton instance = null;
private Singleton() {

}
/**
 * @return
 */
public static Singleton getInstance() {
    if (instance == null) {
        synchronized (Singleton.class) { 
            if (instance == null) { 
                instance=new Singleton();
            }
        }
    }
    return instance;
}
}

这段代码如果instance不加volatile可不可能存在instance=new Singleton();后instance
对其他线程暂时不可见的现象