Poor Mans RCU ver.0.0.1

// Chris M. Thomassons Poor Mans RCU... Example 123...


#include <iostream>
#include <atomic>
#include <thread>
#include <cstdlib>
#include <cstdint>
#include <climits>
#include <functional>


// Masks
static constexpr std::uint32_t ct_ref_mask = 0xFFFFFFF0U;
static constexpr std::uint32_t ct_ref_complete = 0x30U;
static constexpr std::uint32_t ct_ref_inc = 0x20U;
static constexpr std::uint32_t ct_proxy_mask = 0xFU;
static constexpr std::uint32_t ct_proxy_quiescent = 0x10U;


// Iteration settings
static constexpr unsigned long ct_reader_iters_n = 1000000;
static constexpr unsigned long ct_writer_iters_n = 100000;


// Thread counts
static constexpr unsigned long ct_reader_threads_n = 42;
static constexpr unsigned long ct_writer_threads_n = 7;


// Some debug/sanity check things...
// Need to make this conditional in compilation with some macros...
static std::atomic<std::uint32_t> g_debug_node_allocations(0);
static std::atomic<std::uint32_t> g_debug_node_deallocations(0);


// Need to align and pad data structures! To do...

struct ct_node
{
    std::atomic<ct_node*> m_next;
    ct_node* m_defer_next;

    ct_node() : m_next(nullptr), m_defer_next(nullptr)
    {
        g_debug_node_allocations.fetch_add(1, std::memory_order_relaxed);
    }

    ~ct_node()
    {
        g_debug_node_deallocations.fetch_add(1, std::memory_order_relaxed);
    }
};


// The proxy collector itself... :^)
template<std::size_t T_defer_limit>
class ct_proxy
{
    static void prv_destroy(ct_node* n)
    {
        while (n)
        {
            ct_node* next = n->m_defer_next;
            delete n;
            n = next;
        }
    }


public:
    class collector
    {
        friend class ct_proxy;

    private:
        std::atomic<ct_node*> m_defer;
        std::atomic<std::uint32_t> m_defer_count;
        std::atomic<std::uint32_t> m_count;



    public:
        collector()
            : m_defer(nullptr),
            m_defer_count(0),
            m_count(0)
        {

        }


        ~collector()
        {
            prv_destroy(m_defer.load(std::memory_order_relaxed));
        }
    };


private:
    std::atomic<std::uint32_t> m_current;
    std::atomic<bool> m_quiesce;
    ct_node* m_defer;
    collector m_collectors[2];


public:
    ct_proxy()
        : m_current(0),
        m_quiesce(false),
        m_defer(nullptr)
    {

    }

    ~ct_proxy()
    {
        prv_destroy(m_defer);
    }


private:
    void prv_quiesce_begin()
    {
        // Try to begin the quiescence process.
        if (! m_quiesce.exchange(true, std::memory_order_acquire))
        {
            // advance the current collector and grab the old one.
            std::uint32_t old = m_current.load(std::memory_order_relaxed) & ct_proxy_mask;
            old = m_current.exchange((old + 1) & 1, std::memory_order_acq_rel);
            collector& c = m_collectors[old & ct_proxy_mask];

            // decode reference count.
            std::uint32_t refs = old & ct_ref_mask;

            // increment and generate an odd reference count.
            std::uint32_t old_refs = c.m_count.fetch_add(refs + ct_proxy_quiescent, std::memory_order_release);

            if (old_refs == refs)
            {
                // odd reference count and drop-to-zero condition detected!
                prv_quiesce_complete(c);
            }
        }
    }


    void prv_quiesce_complete(collector& c)
    {
        // the collector `c' is now in a quiescent state! :^)
        std::atomic_thread_fence(std::memory_order_acquire);

        // maintain the back link and obtain "fresh" objects from
        // this collection.
        ct_node* n = m_defer;
        m_defer = c.m_defer.load(std::memory_order_relaxed);
        c.m_defer.store(0, std::memory_order_relaxed);

        // reset the reference count.
        c.m_count.store(0, std::memory_order_relaxed);
        c.m_defer_count.store(0, std::memory_order_relaxed);

        // release the quiesce lock.
        m_quiesce.store(false, std::memory_order_release);

        // destroy nodes.
        prv_destroy(n);
    }


public:
    collector& acquire()
    {
        // increment the master count _and_ obtain current collector.
        std::uint32_t current =
            m_current.fetch_add(ct_ref_inc, std::memory_order_acquire);

        // decode the collector index.
        return m_collectors[current & ct_proxy_mask];
    }

    void release(collector& c)
    {
        // decrement the collector.
        std::uint32_t count =
            c.m_count.fetch_sub(ct_ref_inc, std::memory_order_release);

        // check for the completion of the quiescence process.
        if ((count & ct_ref_mask) == ct_ref_complete)
        {
            // odd reference count and drop-to-zero condition detected!
            prv_quiesce_complete(c);
        }
    }


    collector& sync(collector& c)
    {
        // check if the `c' is in the middle of a quiescence process.
        if (c.m_count.load(std::memory_order_relaxed) & ct_proxy_quiescent)
        {
            // drop `c' and get the next collector.
            release(c);

            return acquire();
        }

        return c;
    }


    void collect()
    {
        prv_quiesce_begin();
    }


    void collect(collector& c, ct_node* n)
    {
        if (! n) return;

        // link node into the defer list.
        ct_node* prev = c.m_defer.exchange(n, std::memory_order_relaxed);
        n->m_defer_next = prev;

        // bump the defer count and begin quiescence process if over
        // the limit.
        std::uint32_t count =
            c.m_defer_count.fetch_add(1, std::memory_order_relaxed) + 1;

        if (count >= (T_defer_limit / 2))
        {
            prv_quiesce_begin();
        }
    }
};



typedef ct_proxy<10> ct_proxy_collector;


// you're basic lock-free stack...
// well, minus ABA counter and DWCAS of course!  ;^)
class ct_stack
{
    std::atomic<ct_node*> m_head;


public:
    ct_stack() : m_head(nullptr)
    {

    }


public:
    void push(ct_node* n)
    {
        ct_node* head = m_head.load(std::memory_order_relaxed);

        do
        {
            n->m_next.store(head, std::memory_order_relaxed);
        }

        while (! m_head.compare_exchange_weak(
            head,
            n,
            std::memory_order_release));
    }


    ct_node* flush()
    {
        return m_head.exchange(nullptr, std::memory_order_acquire);
    }


    ct_node* get_head()
    {
        return m_head.load(std::memory_order_acquire);
    }


    ct_node* pop()
    {
        ct_node* head = m_head.load(std::memory_order_acquire);
        ct_node* xchg;

        do
        {
            if (!head) return nullptr;

            xchg = head->m_next.load(std::memory_order_relaxed);
        }

        while (! m_head.compare_exchange_weak(
            head,
            xchg,
            std::memory_order_acquire));

        return head;
    }
};


// The shared state
struct ct_shared
{
    ct_proxy<10> m_proxy_gc;
    ct_stack m_stack;
};



// Reader threads
// Iterates through the lock free stack
void ct_thread_reader(ct_shared& shared)
{
    // iterate the lockfree stack
    for (unsigned long i = 0; i < ct_reader_iters_n; ++i)
    {
        ct_proxy_collector::collector& c = shared.m_proxy_gc.acquire();

        ct_node* n = shared.m_stack.get_head();

        while (n)
        {
            // need to add in some processing...
            //std::this_thread::yield();

            n = n->m_next.load(std::memory_order_relaxed);
        }

        shared.m_proxy_gc.release(c);
    }
}



// Writer threads
// Mutates the lock free stack
void ct_thread_writer(ct_shared& shared)
{
    for (unsigned long wloop = 0; wloop < 42; ++wloop)
    {
        for (unsigned long i = 0; i < ct_writer_iters_n; ++i)
        {
            shared.m_stack.push(new ct_node());
        }

        std::this_thread::yield();

        ct_proxy_collector::collector& c = shared.m_proxy_gc.acquire();

        for (unsigned long i = 0; i < ct_writer_iters_n; ++i)
        {
            shared.m_proxy_gc.collect(c, shared.m_stack.pop());
        }

        shared.m_proxy_gc.release(c);

        std::this_thread::yield();

        if ((wloop % 3) == 0)
        {
            shared.m_proxy_gc.collect();
        }
    }
}



int main()
{
    std::cout << "Chris M. Thomassons Proxy Collector Port ver .0.0.1...\n";
    std::cout << "_______________________________________\n\n";


    {
        ct_shared shared;

        std::thread readers[ct_reader_threads_n];
        std::thread writers[ct_writer_threads_n];

        std::cout << "Booting threads...\n";

        for (unsigned long i = 0; i < ct_writer_threads_n; ++i)
        {
            writers[i] = std::thread(ct_thread_writer, std::ref(shared));
        }

        for (unsigned long i = 0; i < ct_reader_threads_n; ++i)
        {
            readers[i] = std::thread(ct_thread_reader, std::ref(shared));
        }

        std::cout << "Threads running...\n";

        for (unsigned long i = 0; i < ct_reader_threads_n; ++i)
        {
            readers[i].join();
        }

        for (unsigned long i = 0; i < ct_writer_threads_n; ++i)
        {
            writers[i].join();
        }
    }

    std::cout << "Threads completed!\n\n";


    // Sanity check!
    {
        std::uint32_t node_allocations = g_debug_node_allocations.load(std::memory_order_relaxed);
        std::uint32_t node_deallocations = g_debug_node_deallocations.load(std::memory_order_relaxed);

        std::cout << "node_allocations = " << node_allocations << "\n";
        std::cout << "node_deallocations = " << node_deallocations << "\n";

        if (node_allocations != node_deallocations)
        {
            std::cout << "OH SHIT! NODE LEAK!!! SHIT! = " << node_allocations - node_deallocations << "\n\n";
        }

    }

    std::cout << "\n\nTest Completed!\n\n";

    return 0;
}