围绕是否写保证锁在另一个线程新鲜读？ （.NET，内存模型） [英] Does a lock around a write guarantee fresh read in another thread? (.Net, memory model)

问题描述

说我有一个属性，它的制定者是由一个锁保护，但周围没有消气任何锁，如：

Say I have a property whose setter is protected by a lock, but without any lock around the getter, e.g.

private long _myField;
public long MyProperty
{
    get { return _myField; }
    set { lock(whatever) _myField = value; }
}

在除了同步写入（但不读），锁，或者更确切地说Monitor.Exit，应该引起的性写。现在，让我们说，我们有两个线程A和B，以及下列顺序发生：

In addition to synchronizing writes (but not reads), the lock, or rather Monitor.Exit, should cause a volatile write. Let's now say we have two threads A and B, and the following sequence happens:

1. A读取 myProperty的。
的当前值
2. 乙写入一个新值 myProperty的。
3. A读取 myProperty的了。
的当前值

1. A reads the current value of MyProperty.
2. B writes a new value to MyProperty.
3. A reads the current value of MyProperty again.

问：是目前保证看到新的价值？或者是我们的锁只是保证B写入主存储器及时，但不是说其他​​线程读取一个新的价值？或者，可以在答复甚至取决于我们是否在.net 2+或弱ECMA实施运行？

Q: Is A now guaranteed to see the new value? Or did our lock just ensure that B writes to main memory in a timely manner, but not that other threads read a fresh value? Or could the answer even depend on whether we're running in .Net 2+ or a "weaker" ECMA implementation?

推荐答案

没有，因为读没有明确的内存屏障，它不是保证看到新的值。

No, since the read does not have the explicit memory barrier, it is not "guaranteed" to see the new value.

您可以使用 ReaderWriterLockSlim 以确保一）写锁定对方和b）的读取始终皮卡新的价值。

You can use a ReaderWriterLockSlim to insure that a) the writes lock each other and b) the reads always pickup the new value.

private readonly ReaderWriterLockSlim _myFieldLock = new ReaderWriterLockSlim();
private long _myField;
public long MyProperty
{
    get 
    {
        _myFieldLock.EnterReadLock();
        try
        {
            return _myField;
        }
        finally
        {
            _myFieldLock.ExitReadLock();
        }
    }
    set
    {
        _myFieldLock.EnterWriteLock();
        try
        {
            _myField = value;
        }
        finally
        {
            _myFieldLock.ExitWriteLock();
        }
    }
}

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