2.原子

Jun 07, 2016 pm 03:00 PM
cas operate classic use

在运用CAS做Lock-Free操作中有一个经典的ABA问题: 线程1准备用CAS将变量的由A替换为B,在此之前,线程2将变量的由A替换为C,又由C替换为A,然后线程1执行CAS时发现变量的仍然为A,所以CAS成功。但实际上这时的现场已经和最初不同了,尽管CAS成功,但可能存

在运用CAS做Lock-Free操作中有一个经典的ABA问题:

线程1准备用CAS将变量的值由A替换为B,在此之前,线程2将变量的值由A替换为C,又由C替换为A,然后线程1执行CAS时发现变量的值仍然为A,所以CAS成功。但实际上这时的现场已经和最初不同了,尽管CAS成功,但可能存在潜藏的问题,例如下面的例子:

2.原子

现有一个用单向链表实现的堆栈,栈顶为A,这时线程T1已经知道A.next为B,然后希望用CAS将栈顶替换为B:

head.compareAndSet(A,B);

在T1执行上面这条指令之前,线程T2介入,将A、B出栈,再pushD、C、A,此时堆栈结构如下图,而对象B此时处于游离状态:

2.原子

此时轮到线程T1执行CAS操作,检测发现栈顶仍为A,所以CAS成功,栈顶变为B,但实际上B.next为null,所以此时的情况变为:

2.原子

其中堆栈中只有B一个元素,C和D组成的链表不再存在于堆栈中,平白无故就把C、D丢掉了。

以上就是由于ABA问题带来的隐患,各种乐观锁的实现中通常都会用版本戳version来对记录或对象标记,避免并发操作带来的问题,在Java中,AtomicStampedReference也实现了这个作用,它通过包装[E,Integer]的元组来对对象标记版本戳stamp,从而避免ABA问题,例如下面的代码分别用AtomicInteger和AtomicStampedReference来对初始值为100的原子整型变量进行更新,AtomicInteger会成功执行CAS操作,而加上版本戳的AtomicStampedReference对于ABA问题会执行CAS失败:

public class Test {

	private static AtomicInteger atomicInt = new AtomicInteger(100);
	private static AtomicStampedReference atomicStampedRef = new AtomicStampedReference(
			100, 0);
	public static void main(String[] args) throws InterruptedException {
		Thread intT1 = new Thread(new Runnable() {
			@Override
			public void run() {
				atomicInt.compareAndSet(100, 101);
				atomicInt.compareAndSet(101, 100);
			}
		});

		Thread intT2 = new Thread(new Runnable() {
			@Override
			public void run() {
				try {
					TimeUnit.SECONDS.sleep(1);
				} catch (InterruptedException e) {
				}
				boolean c3 = atomicInt.compareAndSet(100, 101);
				System.out.println(c3); // true
			}
		});
		intT1.start();
		intT2.start();
		intT1.join();
		intT2.join();
		Thread refT1 = new Thread(new Runnable() {
			@Override
			public void run() {
				try {
					TimeUnit.SECONDS.sleep(1);
				} catch (InterruptedException e) {

				}
				atomicStampedRef.compareAndSet(100, 101,
						atomicStampedRef.getStamp(),
						atomicStampedRef.getStamp() + 1);

				atomicStampedRef.compareAndSet(101, 100,
						atomicStampedRef.getStamp(),
						atomicStampedRef.getStamp() + 1);

			}
		});

		Thread refT2 = new Thread(new Runnable() {
			@Override
			public void run() {
				int stamp = atomicStampedRef.getStamp();
				try {
					TimeUnit.SECONDS.sleep(2);
				} catch (InterruptedException e) {
				}
				boolean c3 = atomicStampedRef.compareAndSet(100, 101, stamp,
						stamp + 1);
				System.out.println(c3); // false
			}

		});
		refT1.start();
		refT2.start();
	}

}
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