2021.03.15

C++11访存顺序

术语：lockless不使用锁，lock free不会被锁（死锁、活锁）

参考https://en.cppreference.com/w/cpp/atomic/memory_order

Relaxed ordering

memory_order_relaxed

线程间&内无序

Release-Acquire ordering

memory_order_&memory_order_acquire

That is, once the atomic load is completed, thread B is guaranteed to see everything thread A wrote to memory.

The synchronization is established only between the threads releasing and acquiring the same atomic variable. Other threads can see different order of memory accesses than either or both of the synchronized threads.

On strongly-ordered systems — x86, SPARC TSO, IBM mainframe, etc. — release-acquire ordering is automatic for the majority of operations. No additional CPU instructions are issued for this synchronization mode; only certain compiler optimizations are affected (e.g., the compiler is prohibited from moving non-atomic stores past the atomic store-release or performing non-atomic loads earlier than the atomic load-acquire). On weakly-ordered systems (ARM, Itanium, PowerPC), special CPU load or memory fence instructions are used.

Release-Consume ordering

memory_order_release&memory_order_consume

that is, once the atomic load is completed, those operators and functions in thread B that use the value obtained from the load are guaranteed to see what thread A wrote to memory.

注：只有相关的变量才会同步和Release-Acquire的everything区别。

Sequentially-consistent ordering

memory_order_seq_cst

太长了

2021.03.16

LWN关于访存顺序的文章

当时看memory order的c++特性是因为LWN的An introduction to lockless algorithms， 其中一些术语的理解简述如下：

  • "happens before"是多进程指令执行的偏序关系

Articles in this series: （老到新）