Peterson's algorithm in C++11 atomics (Dmitry Vyukov)

peterson.cpp

// Peterson's algorithm in C++11 atomics
//
// Adapted from Dmitry Vyukov's example in the comments to https://bartoszmilewski.com/2008/12/01/c-atomics-and-memory-ordering/.
// For a correctness argument, see https://www.justsoftwaresolutions.co.uk/threading/petersons_lock_with_C++0x_atomics.html.

#include <atomic>
#include <immintrin.h>

constexpr size_t CACHE_LINE_SIZE = 64;
struct alignas(CACHE_LINE_SIZE)
aligned_atomic_bool { std::atomic<bool> val; };
aligned_atomic_bool flag[2];
std::atomic<int> turn = 0;

void lock(int index) {
    const int other_index = 1 - index;
    flag[index].val.store(true, std::memory_order_relaxed);
    turn.exchange(other_index, std::memory_order_acq_rel);

    while (flag[other_index].val.load(std::memory_order_acquire) &&
            other_index == turn.load(std::memory_order_relaxed))
        _mm_pause();
}

void unlock(int index) {
    flag[index].val.store(false, std::memory_order_release);
}