Question

我正在制作视频软件并使用一些现有代码。现有代码包括循环缓冲区。作为制作人，我有一个摄像机和消费者两个不同的线程。一，GLThread，使用OpenGL绘制帧，另一个，VideoCompressorThread，将帧压缩为jpeg格式，以将其保存到视频文件。奇怪的是，目前这两个线程同时处理相同的数据，但这并不会产生竞争条件。在GLThread中我有：

while(!shouldStop) {
        mutex_.lock();
        glw_->makeCurrent();

        shaderProgram_.bind();
        shaderProgram_.setUniformValue("texture", 0);
        shaderProgram_.setAttributeArray("vertex", vertices_.constData());
        shaderProgram_.enableAttributeArray("vertex");
        shaderProgram_.setAttributeArray("textureCoordinate", textureCoordinates_.constData());
        shaderProgram_.enableAttributeArray("textureCoordinate");

        qDebug() << "GLThread: " << "data address: " << static_cast<void*>(imBuf_)  << "time: " << QDateTime::currentMSecsSinceEpoch();
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB8, VIDEO_WIDTH, VIDEO_HEIGHT, 0, GL_RGB, GL_UNSIGNED_BYTE, (GLubyte*)imBuf_);
        qDebug() << "GLThread finished";

        glClear(GL_COLOR_BUFFER_BIT);
        glDrawArrays(GL_TRIANGLES, 0, 6);
        glw_->swapBuffers();

        shaderProgram_.disableAttributeArray("vertex");
        shaderProgram_.disableAttributeArray("textureCoordinate");
        shaderProgram_.release();

        glw_->doneCurrent();
        mutex_.unlock();
}

并在VideoCompressorThread中：

while(!shouldStop)
{
    // JPEG-related stuff
    struct jpeg_compress_struct cinfo;
    struct jpeg_error_mgr       jerr;
    JSAMPROW                    row_pointer;
    unsigned char*              jpgBuf=NULL;
    unsigned long               jpgBufLen=0;

    unsigned char*              data;
    ChunkAttrib                 chunkAttrib;

    // Get raw image from the input buffer
    data = inpBuf->getChunk(&chunkAttrib);

    // Initialize JPEG
    cinfo.err = jpeg_std_error(&jerr);
    jpeg_create_compress(&cinfo);
    jpeg_mem_dest(&cinfo, &jpgBuf, &jpgBufLen);

    // Set the parameters of the output file
    cinfo.image_width = VIDEO_WIDTH;
    cinfo.image_height = VIDEO_HEIGHT;
    cinfo.input_components = 3;
    cinfo.in_color_space = JCS_RGB;

    // Use default compression parameters
    jpeg_set_defaults(&cinfo);
    jpeg_set_quality(&cinfo, jpgQuality, TRUE);

    // Do the compression
    jpeg_start_compress(&cinfo, TRUE);

    // write one row at a time
    qDebug() << "VideoCompressorThread: " << "data address: " << static_cast<void*>(data) << "time: " << QDateTime::currentMSecsSinceEpoch();
    while(cinfo.next_scanline < cinfo.image_height)
    {
        row_pointer = (data + (cinfo.next_scanline * cinfo.image_width * 3));
        jpeg_write_scanlines(&cinfo, &row_pointer, 1);
    }
    qDebug() << "VideoCompressorThread finished";

    // clean up after we're done compressing
    jpeg_finish_compress(&cinfo);


    // Insert compressed image into the output buffer
    chunkAttrib.chunkSize = jpgBufLen;
    outBuf->insertChunk(jpgBuf, chunkAttrib);

    // The output buffer needs to be explicitly freed by the libjpeg client
    free(jpgBuf);
    jpeg_destroy_compress(&cinfo);
}

作为输出我得到：

VideoCompressorThread:  data address:  0x7fffbdcd1060 time:  1438594694479 
VideoCompressorThread finished 
GLThread:  data address:  0x7fffbdcd1060 time:  1438594694488 
GLThread finished 
GLThread:  data address:  0x7fffbddb20b0 time:  1438594694497 
GLThread finished 
VideoCompressorThread:  data address:  0x7fffbddb20b0 time:  1438594694498 
VideoCompressorThread finished 
VideoCompressorThread:  data address:  0x7fffbde93100 time:  1438594694521 
GLThread:  data address:  0x7fffbde93100 time:  1438594694521 
GLThread finished 
VideoCompressorThread finished 
VideoCompressorThread:  data address:  0x7fffbdf74150 time:  1438594694538 
GLThread:  data address:  0x7fffbdf74150 time:  1438594694538 
GLThread finished 
VideoCompressorThread finished 
VideoCompressorThread:  data address:  0x7fffbe0551a0 time:  1438594694555 
GLThread:  data address:  0x7fffbe0551a0 time:  1438594694555 
GLThread finished 
VideoCompressorThread finished 
VideoCompressorThread:  data address:  0x7fffbe1361f0 time:  1438594694571 
GLThread:  data address:  0x7fffbe1361f0 time:  1438594694571 
GLThread finished 
VideoCompressorThread finished 
VideoCompressorThread:  data address:  0x7fffbe217240 time:  1438594694588 
GLThread:  data address:  0x7fffbe217240 time:  1438594694588 
GLThread finished 
VideoCompressorThread finished 
VideoCompressorThread:  data address:  0x7fffbe2f8290 time:  1438594694604 
GLThread:  data address:  0x7fffbe2f8290 time:  1438594694604 
GLThread finished 
VideoCompressorThread finished

正如您所看到的，有时两个线程同时访问相同的数据但没有崩溃。这是纯粹的运气，还是我在这里不明白的东西？我正在使用Xubuntu 14.04，如果这有任何区别。

编辑。 insertChunk和getChunk（）函数。请注意，只有VideoCompressorThread使用getChunk（）获取数据指针。 GLThread连接到chunkReady qt信号。这使得一个主要和多个次要消费者能够使用缓冲区。

void CycDataBuffer::insertChunk(unsigned char* _data, ChunkAttrib &_attrib)
{

    // Check for buffer overflow. CIRC_BUF_MARG is the safety margin against
    // race condition between consumer and producer threads when the buffer
    // is close to full.
    if (buffSemaphore->available() >=  bufSize * (1-CIRC_BUF_MARG))
    {
        cerr << "Circular buffer overflow!" << endl;
        abort();
    }

    // Make sure that the safety margin is at least several (four) times the
    // chunk size. This is necessary to prevent the race condition between
    // consumer and producer threads when the buffer is close to full.
    if(_attrib.chunkSize+sizeof(ChunkAttrib)+MAXLOG > bufSize*MAX_CHUNK_SIZE)
    {
        cerr << "The chunk size is too large!" << endl;
        abort();
    }

    // insert the data into the circular buffer
    _attrib.isRec = isRec;

    memcpy(dataBuf + insertPtr, (unsigned char*)(&_attrib), sizeof(ChunkAttrib));
    insertPtr += sizeof(ChunkAttrib);
    buffSemaphore->release(sizeof(ChunkAttrib));

    memcpy(dataBuf + insertPtr, _data, _attrib.chunkSize);
    buffSemaphore->release(_attrib.chunkSize);

    emit chunkReady(dataBuf + insertPtr);
    insertPtr += _attrib.chunkSize;
    if(insertPtr >= bufSize)
    {
        insertPtr = 0;
    }
}

unsigned char* CycDataBuffer::getChunk(ChunkAttrib* _attrib)
{
    unsigned char* res;

    buffSemaphore->acquire(sizeof(ChunkAttrib));
    memcpy((unsigned char*)_attrib, dataBuf + getPtr, sizeof(ChunkAttrib));
    getPtr += sizeof(ChunkAttrib);

    buffSemaphore->acquire(_attrib->chunkSize);
    res = dataBuf + getPtr;

    getPtr += _attrib->chunkSize;
    if(getPtr >= bufSize)
    {
        getPtr = 0;
    }

    return(res);
}

Answer 1

仅仅因为它没有崩溃并不意味着它不是一个错误。在另一个线程正在读取时写入缓冲区通常会导致数据损坏由阅读线程阅读。有些字节读取为新值，有些则为旧值。

您将看到的事情之一是图像缓冲区的某些部分被覆盖而另一个线程正在处理它，这会在观看视频时产生screen-tearing。你可以通过在屏幕上快速移动的斜条纹看到最佳效果。

读取相同缓冲区的2个线程非常精细，当有人开始写入问题时就开始了。

Answer 2

除了棘手的怪异答案＆＃34;仅仅因为它没有崩溃并不意味着它不是一个错误＆＃34;下面，我想补充一点，我实际上并没有看到为什么这两个特定的代码片段无法并行工作的原因。两者都只读取相同的图像数据，这非常好。

只有在至少有两个线程（或共享内存的进程）访问同一个缓冲区并且至少有一个线程正在修改它时，即通过覆盖数据时，才会出现并发访问缓冲区的问题，或者通过取消分配缓冲区。

jpeg_write_scanlines和glTexImage2D线程安全。为什么不崩溃？

2 个答案: