# “Do you know what is read-copy-update?”

Balázs randomly mumured on a Friday afternoon while I was doom scrolling the endless stream of despair on Memegen some time shortly after the Jan 2023 layoffs. Friday wasn’t one of the team designated RTO days, but I decided to work from office anyway for some focus time, as few people would be coming in.

B: “It’s pretty cool.”

Balázs proceeded to explain to me that read-copy-update, or RCU, is a lock-free mechanism to update data structures that are actively being consumed by asynchronous readers without the usage of locks. It was first introduced into the Linux kernel back in 2002, but discussions and designs date back to the mid 90s. The general idea is that instead of applying a mutex lock to update an asynchronously read data structure, instantiate a copy of the existing data structure and apply changes to the new instance. After the changes are applied, do an atomic update of the pointer from the old instance to the new one. This essentially creates a lock-free update of data change.