Using LKMM atomics in Rust

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2024-10-20 13:00:08

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Rust, like C, has its own memory model describing how concurrent access to the same data by multiple threads can behave. The Linux kernel, however, has its own ideas. The Linux kernel memory model (LKMM) is subtly different from both the standard C memory model and Rust's model. At Kangrejos, Boqun Feng gave a presentation about the need to reconcile the memory models used by Rust and the kernel, including a few potential avenues for doing so. While no consensus was reached, it is an area of active discussion.

The problem, Feng explained, is that the LKMM makes guarantees that the Rust memory model does not. Since the compiler doesn't know about those guarantees, it can (potentially) make optimizations that break them. The only saving grace is the ABI between C and Rust code, which should have certain guarantees that both sides are aware of. However, in practice, many architectures don't specify any guarantees about atomic operations or interactions between threads as part of their ABI. Having an ABI that listed the relevant guarantees wouldn't be a complete solution in any case — cross-language link-time-optimization (LTO) could still cause problems, Feng said.