serenity/LibGUI/GLock.h

#pragma once

#include <AK/Assertions.h>
#include <AK/Types.h>
#include <unistd.h>

#define memory_barrier() asm volatile ("" ::: "memory")

static inline dword CAS(volatile dword* mem, dword newval, dword oldval)
{
    dword ret;
    asm volatile(
        "cmpxchgl %2, %1"
        :"=a"(ret), "+m"(*mem)
        :"r"(newval), "0"(oldval)
        :"cc", "memory");
    return ret;
}

class GLock {
public:
    GLock() { }
    ~GLock() { }

    void lock();
    void unlock();

private:
    volatile dword m_lock { 0 };
    dword m_level { 0 };
    int m_holder { -1 };
};

class GLocker {
public:
    [[gnu::always_inline]] inline explicit GLocker(GLock& l) : m_lock(l) { lock(); }
    [[gnu::always_inline]] inline ~GLocker() { unlock(); }
    [[gnu::always_inline]] inline void unlock() { m_lock.unlock(); }
    [[gnu::always_inline]] inline void lock() { m_lock.lock(); }

private:
    GLock& m_lock;
};

[[gnu::always_inline]] inline void GLock::lock()
{
    for (;;) {
        if (CAS(&m_lock, 1, 0) == 0) {
            if (m_holder == -1 || m_holder == gettid()) {
                m_holder = gettid();
                ++m_level;
                memory_barrier();
                m_lock = 0;
                return;
            }
            m_lock = 0;
        }
        donate(m_holder);
    }
}

inline void GLock::unlock()
{
    for (;;) {
        if (CAS(&m_lock, 1, 0) == 0) {
            ASSERT(m_holder == gettid());
            ASSERT(m_level);
            --m_level;
            if (m_level) {
                memory_barrier();
                m_lock = 0;
                return;
            }
            m_holder = -1;
            memory_barrier();
            m_lock = 0;
            return;
        }
        donate(m_holder);
    }
}

#define LOCKER(lock) GLocker locker(lock)

template<typename T>
class GLockable {
public:
    GLockable() { }
    GLockable(T&& resource) : m_resource(move(resource)) { }
    GLock& lock() { return m_lock; }
    T& resource() { return m_resource; }

    T lock_and_copy()
    {
        LOCKER(m_lock);
        return m_resource;
    }

private:
    T m_resource;
    GLock m_lock;
};