OpenGL resource loading on second thread

Question

I use win32 threads with OpenGL 2.1. What I'm trying to achieve is to render simple image saying "loading", while in the background whole 3D scene is being loaded. It works now, but I have a problem, where sometimes a part of my cubemap texture takes data from Mozilla Firefox Browser (How in the hell does this happen???) and ignore that small box, with texture, it's only a sprite and it is where it should be.:

This happens like 1 in 3 times I try to load my program. This is how my thread looks:

WindowsThread::WindowsThread(HGLRC graphicsContext, HDC deviceContext) :
    graphicsContext_(graphicsContext),
    deviceContext_(deviceContext),
    running_(false),
    task_(0),
    mode_(WT_NORMAL)
{
    handle_ = CreateThread(0, 0,
        (unsigned long (__stdcall *)(void *)) this->staticRun,
        (void*) this, CREATE_SUSPENDED, &id_);

    if (handle_ == 0) {
        LOGE("Unable to create thread.");
        return;
    }

    if (!SetThreadPriority(handle_, THREAD_PRIORITY_NORMAL)) {
        LOGE("Unable to set thread priority for thread.");
        return;
    }
}

WindowsThread::~WindowsThread() {
    finishTask();
    running_ = false;
    WaitForSingleObject(handle_, INFINITE);
    CloseHandle(handle_);
    wglDeleteContext(graphicsContext_);
}

void WindowsThread::start() {
    running_ = true;
    if (!ResumeThread(handle_)) {
        LOGW("Unable to resume thread.");
    }
}

bool WindowsThread::isRunning() {
    return running_;
}

void WindowsThread::setTask(Task* task, Mode mode) {
    finishTask();
    task_ = task;
    mode_ = mode;
}

bool WindowsThread::hasTask() {
    return task_ != 0;
}

void WindowsThread::finishTask() {
    while (task_ != 0) {
        Sleep(1);
    }
}

void WindowsThread::stop() {
    running_ = false;
}

int WindowsThread::staticRun(void* thread) {
    return ((WindowsThread*) thread)->run();
}

int WindowsThread::run() {
    wglMakeCurrent(deviceContext_, graphicsContext_);
    while (running_) {
        if (task_ != 0) {
            task_->run();
            task_ = 0;
        }
        Sleep(10);
    }
    wglMakeCurrent(0, 0);
    return 1;
}

ThreadManager:

WindowsThreadManager::WindowsThreadManager(
    System* system, UINT threadPoolSize)
{
    if (threadPoolSize == 0) {
        SYSTEM_INFO info;
        GetSystemInfo(&info);
        threadPoolSize = info.dwNumberOfProcessors;
        if (threadPoolSize == 0) {
            threadPoolSize = 1;
        }
    }
    LOGI("Number of threads used: %d", threadPoolSize);
    masterContext_ = wglGetCurrentContext();
    HDC hdc = wglGetCurrentDC();
    for (UINT i = 0; i < threadPoolSize; i++) {
        HGLRC threadContext = wglCreateContext(hdc);
        wglShareLists(masterContext_, threadContext);
        WindowsThread* thread = new WindowsThread(threadContext, hdc);
        thread->start();
        threads_.push_back(thread);
    }
}

WindowsThreadManager::~WindowsThreadManager() {
    for (UINT i = 0; i < threads_.size(); i++) {
        delete threads_[i];
    }
    for (UINT i = 0; i < tasks_.size(); i++) {
        delete tasks_[i];
    }
}

void WindowsThreadManager::execute(Task* task, Mode mode) {
    WindowsThread::Mode wtMode = WindowsThread::WT_NORMAL;
    if (mode == TM_GRAPHICS_CONTEXT) {
        wtMode = WindowsThread::WT_GRPAHICS_CONTEXT;
    }
    tasks_.push_back(task);
    for (UINT i = 0; i < threads_.size(); i++) {
        if (!threads_[i]->hasTask()) {
            threads_[i]->setTask(task, wtMode);
            return;
        }
    }
    threads_[0]->setTask(task, wtMode);
}

void WindowsThreadManager::joinAll() {
    for (UINT i = 0; i < threads_.size(); i++) {
        if (threads_[i]->hasTask()) {
            threads_[i]->finishTask();
        }
    }
}

I use Nvidia 670GTX with latest drivers on Winodws 8. Any Ideas where the problem might be?

[EDIT] I added glFinish() at the end of my loader thread, and now everything loads normaly. I red somewhere, that OpenGL doesn't immediately finish all of it's work, so I guess this was the case, where context was set to NULL before it could finish it's work.

Answer 1

It works now, but I have a problem, where sometimes a part of my cubemap texture takes data from Mozilla Firefox Browser (How in the hell does this happen???)

Your texture receives data from uninitialized graphics memory, which very likely may contain residual images from another process, that previously used that memory region. Stuff like this can happen, if

a) the driver has a bug and doesn't synchronize resources between threads

and

b) if you're trying to modify a texture, while it's bound to a texture unit in the other thread.

EDIT: You can (and should) introduce proper synchronization yourself. Simply because it enhances performance. Use condition variables to communicate between threads, when the texture is currently not busy. Ideally you use two or more textures, you update in a round robin fashion.