How to implement C++26 RCU

Denis Yaroshevskiy

This talk

• TODO LINK
• Paul McKenney, Yedidya Feldblum
• motivational example
• trivial buggy implementation
• lowlevel concurrency in C++
• improvements to RCU

Motivational Example

std::shared_ptr + mutex

Atomic Shared Ptr

Raw pointer

Using RCU

implementing basic rcu

Testing using relacy

• github
• Dmitry Vyukov

RCU smoke test

RCU (broken)

Bug report

low level concurrency in c++

Approaches to Understanding

• C++ standard
• Understand hardware first
• Made up hardware model

What does relaxed do?

• "atomicity"
• while (!done.load(...))

What do acquire / release do?

• release store waits for previous stores to leave the buffer
• acquire load disallows the following reads to come from the buffer
• nothing in the speicification looks like what I said
• [atomics.order] release ... synchronizes with ... acquire ... takes its value

RCU test