The concept of Cache is introduced in contemporary computer multi-core CPU architecture, such as SMP (symmetric multiprocessor) architecture, and the introduction of Cache will cause some other problems. When we load the data required by a CPU into the cache closest to the CPU, and at the same time When another CPU needs to use the same data, it will cause data consistency problems, and memory barriers are a way to solve these problems.
Highly recommend reading the article "Memory Barriers: a Hardware View for Software Hackers"
http://ifeve.com/tag/memory-barriers/#memory-barriers