[QOpenGLWidget] flashing image where I use OpenCL

[debian]

Hello!
I have to output several network streams, now I use qt-5.5.1 and OpenGL.
Because network streams in YUV420 I convert it to RGB (http://www.fourcc.org/fccyvrgb.php), it work, but not very quick.
Now I try use OpenCL for convert streams, follow my code:
QImage OCL::convert(const char *buf, quint16 width, quint16 height, ocl_queue queue)
{
QImage img;
if (_ready && buf && width && height && queue < QUEUE_UNKNOWN) {
const size_t stride = width * 4;
const size_t sz = stride * height;
const size_t dims[2] = {width, height};
cl_mem in = NULL;
cl_mem out = NULL;
_rgb[queue].resize(sz);
in = clCreateBuffer(_context, CL_MEM_READ_ONLY, width * height * 3 / 2, NULL, &_ret);
if (IS_ERROR(_ret)) {
qDebug() << "clCreateBuffer input, error" << _ret;
goto EXIT;
}
out = clCreateBuffer(_context, CL_MEM_WRITE_ONLY, sz, NULL, &_ret);
if (IS_ERROR(_ret)) {
qDebug() << "clCreateBuffer out, error" << _ret;
goto EXIT;
}
_ret = clSetKernelArg(_kernel, 0, sizeof(cl_mem), &in);
if (IS_ERROR(_ret)) {
qDebug() << "clSetKernelArg 0, error" << _ret;
goto EXIT;
}
_ret = clSetKernelArg(_kernel, 1, sizeof(cl_mem), &out);
if (IS_ERROR(_ret)) {
qDebug() << "clSetKernelArg 1, error" << _ret;
goto EXIT;
}
_ret = clEnqueueWriteBuffer(_command[queue], in, CL_TRUE, 0, width * height * 3 / 2, buf, 0, NULL, NULL);
if (IS_ERROR(_ret)) {
qDebug() << "clEnqueueWriteBuffer, error" << _ret;
goto EXIT;
}
// FIXME: periodic clEnqueueNDRangeKernel, error -52
_ret = clEnqueueNDRangeKernel(_command[queue], _kernel, 2, NULL, dims, NULL, 0, NULL, NULL);
if (IS_ERROR(_ret)) {
qDebug() << "clEnqueueNDRangeKernel, error" << _ret;
goto EXIT;
}
_ret = clEnqueueReadBuffer(_command[queue], out, CL_TRUE, 0, sz, _rgb[queue].data(), 0, NULL, NULL);
if (IS_ERROR(_ret)) {
qDebug() << "clEnqueueReadBuffer, error" << _ret;
goto EXIT;
}
img = QImage(reinterpret_cast<const uchar *>(_rgb[queue].constData()), width, height, stride, QImage::Format_ARGB32);
EXIT:
if (in != NULL)
clReleaseMemObject(in);
if (out != NULL)
clReleaseMemObject(out);
}
return img;
}
This is work very quick, but I see flashing images.
How can I solve this problem?
Thank you and excuse my bad english.