unfair_mutex_impl.hpp

// Copyright Take Vos 2020-2022.
// 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 "unfair_mutex_intf.hpp"
#include "global_state.hpp"
#include "../utility/utility.hpp"
#include "../macros.hpp"
#include <atomic>
#include <memory>
#include <format>
 
hi_export_module(hikogui.concurrency.unfair_mutex : impl);
 
namespace hi { inline namespace v1 {
namespace detail {
 
inline unfair_mutex_impl<false> unfair_mutex_deadlock_mutex;
 
thread_local inline std::vector<void *> unfair_mutex_deadlock_stack;
 
inline std::vector<std::pair<void *, void *>> unfair_mutex_deadlock_lock_graph;
 
[[nodiscard]] inline void *unfair_mutex_deadlock_check_graph(void *object) noexcept
{
    hi_assert_not_null(object);
 
    hilet lock = std::scoped_lock(detail::unfair_mutex_deadlock_mutex);
 
    for (hilet before : unfair_mutex_deadlock_stack) {
        auto correct_order = std::make_pair(before, object);
        hilet reverse_order = std::make_pair(object, before);
 
        if (std::binary_search(
                unfair_mutex_deadlock_lock_graph.cbegin(), unfair_mutex_deadlock_lock_graph.cend(), correct_order)) {
            // The object has been locked in the correct order in comparison to `before`.
            continue;
        }
 
        if (std::binary_search(
                unfair_mutex_deadlock_lock_graph.cbegin(), unfair_mutex_deadlock_lock_graph.cend(), reverse_order)) {
            // The object has been locked in reverse order in comparison to `before`.
            return before;
        }
 
        // Insert the new 'correct' order in the sorted lock_graph.
        hilet it =
            std::upper_bound(unfair_mutex_deadlock_lock_graph.cbegin(), unfair_mutex_deadlock_lock_graph.cend(), correct_order);
        unfair_mutex_deadlock_lock_graph.insert(it, std::move(correct_order));
    }
    return nullptr;
}
 
} // namespace detail
 
hi_export inline void *unfair_mutex_deadlock_lock(void *object) noexcept
{
    if (is_system_shutting_down()) {
        // thread_local variables used by `stack` do not work on MSVC after main() returns.
        return nullptr;
    }
 
    hi_assert_not_null(object);
 
    if (std::count(detail::unfair_mutex_deadlock_stack.begin(), detail::unfair_mutex_deadlock_stack.end(), object) != 0) {
        // `object` already locked by the current thread.
        return object;
    }
 
    if (auto before = detail::unfair_mutex_deadlock_check_graph(object)) {
        // Trying to lock `object` after `before` in previously reversed order
        return before;
    }
 
    detail::unfair_mutex_deadlock_stack.push_back(object);
    return nullptr;
}
 
hi_export inline bool unfair_mutex_deadlock_unlock(void *object) noexcept
{
    if (is_system_shutting_down()) {
        // thread_local variables used by `stack` do not work on MSVC when main() returns.
        return true;
    }
 
    hi_assert_not_null(object);
 
    // Trying to unlock `object`, but nothing on this thread was locked.
    if (detail::unfair_mutex_deadlock_stack.empty()) {
        return false;
    }
 
    // Trying to unlock `object`, but unlocking in different order.
    if (detail::unfair_mutex_deadlock_stack.back() != object) {
        return false;
    }
 
    detail::unfair_mutex_deadlock_stack.pop_back();
    return true;
}
 
hi_export inline void unfair_mutex_deadlock_remove_object(void *object) noexcept
{
    hi_assert_not_null(object);
 
    if (is_system_shutting_down()) {
        // thread_local variables used by `lock_graph` do not work on MSVC when main() returns.
        return;
    }
 
    hilet lock = std::scoped_lock(detail::unfair_mutex_deadlock_mutex);
    std::erase_if(detail::unfair_mutex_deadlock_lock_graph, [object](hilet& item) {
        return item.first == object or item.second == object;
    });
}
 
hi_export inline void unfair_mutex_deadlock_clear_stack() noexcept
{
    if (is_system_shutting_down()) {
        // thread_local variables used by `stack` do not work on MSVC when main() returns.
        return;
    }
 
    detail::unfair_mutex_deadlock_stack.clear();
}
 
hi_export inline void unfair_mutex_deadlock_clear_graph() noexcept
{
    if (is_system_shutting_down()) {
        // thread_local variables used by `lock_graph` do not work on MSVC when main() returns.
        return;
    }
 
    hilet lock = std::scoped_lock(detail::unfair_mutex_deadlock_mutex);
    detail::unfair_mutex_deadlock_lock_graph.clear();
}
 
template<bool UseDeadLockDetector>
inline unfair_mutex_impl<UseDeadLockDetector>::~unfair_mutex_impl()
{
    hi_axiom(not is_locked());
    if constexpr (UseDeadLockDetector) {
        unfair_mutex_deadlock_remove_object(this);
    }
}
 
template<bool UseDeadLockDetector>
inline bool unfair_mutex_impl<UseDeadLockDetector>::is_locked() const noexcept
{
    return semaphore.load(std::memory_order::relaxed) != 0;
}
 
template<bool UseDeadLockDetector>
inline void unfair_mutex_impl<UseDeadLockDetector>::lock() noexcept
{
    if constexpr (UseDeadLockDetector) {
        hilet other = unfair_mutex_deadlock_lock(this);
        hi_assert(other != this, "This mutex is already locked.");
        hi_assert(other == nullptr, "Potential dead-lock because of different lock ordering of mutexes.");
    }
 
    hi_axiom(holds_invariant());
 
    // Switch to 1 means there are no waiters.
    semaphore_value_type expected = 0;
    if (not semaphore.compare_exchange_strong(expected, 1, std::memory_order::acquire)) {
        [[unlikely]] lock_contended(expected);
    }
 
    hi_axiom(holds_invariant());
}
 
template<bool UseDeadLockDetector>
[[nodiscard]] inline bool unfair_mutex_impl<UseDeadLockDetector>::try_lock() noexcept
{
    if constexpr (UseDeadLockDetector) {
        hilet other = unfair_mutex_deadlock_lock(this);
        hi_assert(other != this, "This mutex is already locked.");
        hi_assert(other == nullptr, "Potential dead-lock because of different lock ordering of mutexes.");
    }
 
    hi_axiom(holds_invariant());
 
    // Switch to 1 means there are no waiters.
    semaphore_value_type expected = 0;
    if (not semaphore.compare_exchange_strong(expected, 1, std::memory_order::acquire)) {
        hi_axiom(holds_invariant());
 
        if constexpr (UseDeadLockDetector) {
            hi_assert(unfair_mutex_deadlock_unlock(this), "Unlock is not done in reverse order.");
        }
 
        [[unlikely]] return false;
    }
 
    hi_axiom(holds_invariant());
    return true;
}
 
template<bool UseDeadLockDetector>
inline void unfair_mutex_impl<UseDeadLockDetector>::unlock() noexcept
{
    if constexpr (UseDeadLockDetector) {
        hi_assert(unfair_mutex_deadlock_unlock(this), "Unlock is not done in reverse order.");
    }
 
    hi_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);
    }
 
    hi_axiom(holds_invariant());
}
 
template<bool UseDeadLockDetector>
[[nodiscard]] inline bool unfair_mutex_impl<UseDeadLockDetector>::holds_invariant() const noexcept
{
    return semaphore.load(std::memory_order::relaxed) <= 2;
}
 
template<bool UseDeadLockDetector>
hi_no_inline inline void unfair_mutex_impl<UseDeadLockDetector>::lock_contended(semaphore_value_type expected) noexcept
{
    hi_axiom(holds_invariant());
 
    do {
        hilet should_wait = expected == 2;
 
        // Set to 2 when we are waiting.
        expected = 1;
        if (should_wait || semaphore.compare_exchange_strong(expected, 2)) {
            hi_axiom(holds_invariant());
            semaphore.wait(2);
        }
 
        hi_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));
}
 
}} // namespace hi::v1