spinlock.h

/*
    This file is part of rippled: https://github.com/ripple/rippled
    Copyright 2022, Nikolaos D. Bougalis <nikb@bougalis.net>
 
    Permission to use, copy, modify, and/or distribute this software for any
    purpose  with  or without fee is hereby granted, provided that the above
    copyright notice and this permission notice appear in all copies.
 
    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
 
#ifndef RIPPLE_BASICS_SPINLOCK_H_INCLUDED
#define RIPPLE_BASICS_SPINLOCK_H_INCLUDED
 
#include <atomic>
#include <cassert>
#include <limits>
#include <type_traits>
 
#ifndef __aarch64__
#include <immintrin.h>
#endif
 
namespace ripple {
 
namespace detail {
inline void
spin_pause() noexcept
{
#ifdef __aarch64__
    asm volatile("yield");
#else
    _mm_pause();
#endif
}
 
}  // namespace detail
 
template <class T>
class packed_spinlock
{
    // clang-format off
    static_assert(std::is_unsigned_v<T>);
    static_assert(std::atomic<T>::is_always_lock_free);
    static_assert(
        std::is_same_v<decltype(std::declval<std::atomic<T>&>().fetch_or(0)), T> &&
        std::is_same_v<decltype(std::declval<std::atomic<T>&>().fetch_and(0)), T>,
        "std::atomic<T>::fetch_and(T) and std::atomic<T>::fetch_and(T) are required by packed_spinlock");
    // clang-format on
 
private:
    std::atomic<T>& bits_;
    T const mask_;
 
public:
    packed_spinlock(packed_spinlock const&) = delete;
    packed_spinlock&
    operator=(packed_spinlock const&) = delete;
 
    packed_spinlock(std::atomic<T>& lock, int index)
        : bits_(lock), mask_(static_cast<T>(1) << index)
    {
        assert(index >= 0 && (mask_ != 0));
    }
 
    [[nodiscard]] bool
    try_lock()
    {
        return (bits_.fetch_or(mask_, std::memory_order_acquire) & mask_) == 0;
    }
 
    void
    lock()
    {
        while (!try_lock())
        {
            // The use of relaxed memory ordering here is intentional and
            // serves to help reduce cache coherency traffic during times
            // of contention by avoiding writes that would definitely not
            // result in the lock being acquired.
            while ((bits_.load(std::memory_order_relaxed) & mask_) != 0)
                detail::spin_pause();
        }
    }
 
    void
    unlock()
    {
        bits_.fetch_and(~mask_, std::memory_order_release);
    }
};
 
template <class T>
class spinlock
{
    static_assert(std::is_unsigned_v<T>);
    static_assert(std::atomic<T>::is_always_lock_free);
 
private:
    std::atomic<T>& lock_;
 
public:
    spinlock(spinlock const&) = delete;
    spinlock&
    operator=(spinlock const&) = delete;
 
    spinlock(std::atomic<T>& lock) : lock_(lock)
    {
    }
 
    [[nodiscard]] bool
    try_lock()
    {
        T expected = 0;
 
        return lock_.compare_exchange_weak(
            expected,
            std::numeric_limits<T>::max(),
            std::memory_order_acquire,
            std::memory_order_relaxed);
    }
 
    void
    lock()
    {
        while (!try_lock())
        {
            // The use of relaxed memory ordering here is intentional and
            // serves to help reduce cache coherency traffic during times
            // of contention by avoiding writes that would definitely not
            // result in the lock being acquired.
            while (lock_.load(std::memory_order_relaxed) != 0)
                detail::spin_pause();
        }
    }
 
    void
    unlock()
    {
        lock_.store(0, std::memory_order_release);
    }
};
}  // namespace ripple
 
#endif