在没有锁定的情况下读取同时被修改的整数变量是否安全? [英] Is it safe to read an integer variable that's being concurrently modified without locking?

问题描述

假设我在一个类中有一个整型变量，这个变量可能会被其他线程并发修改.写入受互斥锁保护.我也需要保护读取吗?我听说有一些硬件架构，如果一个线程修改一个变量，另一个线程读取它，那么读取结果将是垃圾；在这种情况下，我确实需要保护读取.不过，我从未见过这样的架构.

Suppose that I have an integer variable in a class, and this variable may be concurrently modified by other threads. Writes are protected by a mutex. Do I need to protect reads too? I've heard that there are some hardware architectures on which, if one thread modifies a variable, and another thread reads it, then the read result will be garbage; in this case I do need to protect reads. I've never seen such architectures though.

此问题假设单个事务仅包含更新单个整数变量，因此我不担心也可能涉及事务的任何其他变量的状态.

This question assumes that a single transaction only consists of updating a single integer variable so I'm not worried about the states of any other variables that might also be involved in a transaction.

推荐答案

原子读
如前所述，它依赖于平台.在 x86 上，该值必须在 4 字节边界上对齐.通常对于大多数平台，读取必须在单个 CPU 指令中执行.

atomic read
As said before, it's platform dependent. On x86, the value must be aligned on a 4 byte boundary. Generally for most platforms, the read must execute in a single CPU instruction.

优化器缓存
优化器不知道您正在读取由不同线程修改的值.声明值 volatile 有助于:优化器将为每次访问发出内存读/写，而不是尝试将值缓存在寄存器中.

optimizer caching
The optimizer doesn't know you are reading a value modified by a different thread. declaring the value volatile helps with that: the optimizer will issue a memory read / write for every access, instead of trying to keep the value cached in a register.

CPU 缓存
不过，您可能会读取一个陈旧的值，因为在现代架构中，您有多个内核和单独的缓存，这些缓存不会自动保持同步.您需要一个读内存屏障，通常是一个特定于平台的指令.

CPU cache
Still, you might read a stale value, since on modern architectures you have multiple cores with individual cache that is not kept in sync automatically. You need a read memory barrier, usually a platform-specific instruction.

在 Wintel 上，线程同步函数会自动添加一个完整的内存屏障，或者您可以使用 InterlockedXxxx 函数.

On Wintel, thread synchronization functions will automatically add a full memory barrier, or you can use the InterlockedXxxx functions.

MSDN:内存和同步问题，MemoryBarrier 宏

[edit] 另请参阅 drhirsch 的评论.

[edit] please also see drhirsch's comments.

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