A lock-free atomic shared_ptr - Timur Doumler - NDC TechTown 2022

std : : shared_ptr is a standard smart pointer utility widely used in modern C++. A commonly overlooked property of std : : shared_ptr is that while its control block is thread-safe, the shared_ptr itself isn't. To fix this, C++20 introduced std : : atomic std : : shared_ptr . However, existing implementations are typically not lock-free, rendering std :: atomic std::shared_ptr useless for low-latency and real-time applications.
What would it take to implement a lock-free atomic shared_ptr? In this talk, we first discuss the motivation and use case, review how std :: shared_ptr works and the history of std :: atomic std::shared_ptr. We then look at existing implementations and different implementation strategies. Finally, we present a new implementation of a lock-free atomic shared_ptr which is portable to multiple platforms.
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[@OlliS71]

It doesn't matter if DWCAS is 2x slower. The time for the DWCAS doesn't count. The real cost comes from invalidating other caches sharing the same cacheline for the DWCAS'd word. Cache 2 cache-transfers are usually very slow.