libcruft-util/cruft/util/thread/spinlock.hpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2018-2019 Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include <atomic>

namespace cruft::thread {
    /// A CPU intensive, but lower latency, lock.
    ///
    /// std::atomic_flag seems like it might have been a good option on which
    /// to base our implementation, but it appears potentially expensive to
    /// spin over without a cheap read operation.
    ///
    /// Satisfies BasicLockable.
    ///
    /// NOTE: All move operations assume that either the lock is held by the
    ///  client performing the move, or that is not held by any client.
    class spinlock {
    public:
        spinlock ();
        ~spinlock ();

        spinlock (spinlock &&) noexcept;
        spinlock& operator= (spinlock &&) noexcept;

        spinlock (const spinlock&) = delete;
        spinlock& operator= (const spinlock&) = delete;

        void lock ();
        void unlock ();

    private:
        static_assert (
            ::std::atomic<bool>::is_always_lock_free,
            "we require a lock free primitive or the entire point is moot"
        );

        ::std::atomic<bool> held;
    };
};