unfair_mutex.hpp

// Copyright Take Vos 2020.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
 
#pragma once
 
#include "required.hpp"
#include "thread.hpp"
#include "assert.hpp"
#include "dead_lock_detector.hpp"
#include <atomic>
#include <memory>
#include <thread>
 
namespace tt {
 
template<bool UseDeadLockDetector>
class unfair_mutex_impl {
public:
    constexpr unfair_mutex_impl() noexcept {}
    unfair_mutex_impl(unfair_mutex_impl const &) = delete;
    unfair_mutex_impl(unfair_mutex_impl &&) = delete;
    unfair_mutex_impl &operator=(unfair_mutex_impl const &) = delete;
    unfair_mutex_impl &operator=(unfair_mutex_impl &&) = delete;
 
    ~unfair_mutex_impl() requires (UseDeadLockDetector)
    {
        dead_lock_detector::remove_object(this);
    }
 
    ~unfair_mutex_impl() = default;
 
    bool is_locked() const noexcept
    {
        return semaphore.load(std::memory_order::relaxed) != 0;
    }
 
    void lock() noexcept
    {
        if constexpr (UseDeadLockDetector) {
            ttlet other = dead_lock_detector::lock(this);
            tt_axiom(other != this, "Mutex already locked.");
            tt_axiom(other == nullptr, "Potential dead-lock.");
        }
 
        tt_axiom(holds_invariant());
 
        // Switch to 1 means there are no waiters.
        semaphore_value_type expected = 0;
        if (!semaphore.compare_exchange_strong(expected, 1, std::memory_order::acquire)) {
            [[unlikely]] lock_contended(expected);
        }
 
        tt_axiom(holds_invariant());
    }
 
    [[nodiscard]] bool try_lock() noexcept {
        if constexpr (UseDeadLockDetector) {
            ttlet other = dead_lock_detector::lock(this);
            tt_axiom(other != this, "Mutex already locked.");
            tt_axiom(other == nullptr, "Potential dead-lock.");
        }
 
        tt_axiom(holds_invariant());
 
        // Switch to 1 means there are no waiters.
        semaphore_value_type expected = 0;
        if (!semaphore.compare_exchange_strong(expected, 1, std::memory_order::acquire)) {
            tt_axiom(holds_invariant());
 
            if constexpr (UseDeadLockDetector) {
                tt_axiom(dead_lock_detector::unlock(this), "Unlocking mutex out of order.");
            }
 
            [[unlikely]] return false;
        }
 
        tt_axiom(holds_invariant());
        return true;
    }
 
    void unlock() noexcept {
        if constexpr (UseDeadLockDetector) {
            tt_axiom(dead_lock_detector::unlock(this), "Unlocking mutex out of order.");
        }
 
        tt_axiom(holds_invariant());
 
        if (semaphore.fetch_sub(1, std::memory_order::relaxed) != 1) {
            [[unlikely]] semaphore.store(0, std::memory_order::release);
 
            semaphore.notify_one();
        } else {
            atomic_thread_fence(std::memory_order::release);
        }
 
        tt_axiom(holds_invariant());
    }
 
private:
    /*
     * semaphore value:
     *  0 - Unlocked, no other thread is waiting.
     *  1 - Locked, no other thread is waiting.
     *  2 - Locked, zero or more threads are waiting.
     */
    std::atomic_unsigned_lock_free semaphore = 0;
    using semaphore_value_type = typename decltype(semaphore)::value_type;
 
    bool holds_invariant() const noexcept
    {
        return semaphore.load(std::memory_order::relaxed) <= 2;
    }
 
    tt_no_inline void lock_contended(semaphore_value_type expected) noexcept
    {
        tt_axiom(holds_invariant());
 
        do {
            ttlet should_wait = expected == 2;
 
            // Set to 2 when we are waiting.
            expected = 1;
            if (should_wait || semaphore.compare_exchange_strong(expected, 2)) {
                tt_axiom(holds_invariant());
                semaphore.wait(2);
            }
 
            tt_axiom(holds_invariant());
            // Set to 2 when acquiring the lock, so that during unlock we wake other waiting threads.
            expected = 0;
        } while (!semaphore.compare_exchange_strong(expected, 2));
    }
};
 
#if TT_BUILD_TYPE == TT_BT_DEBUG
using unfair_mutex = unfair_mutex_impl<true>;
#else
using unfair_mutex = unfair_mutex_impl<false>;
#endif
 
}