thread locking around avcodec_open/close

I have a c++-cli code that is capturing videos from a folder in opencv using capture and then retrieving frames using cvquery frame. I then process frames and once all frames are processed i release the capture. It works fine but when I try to multithread it gives me a warning and cannot capture some of the videos in the folder with warning " insufficient thread locking around avcodec_open/close()".

//for each video in folder do
{
    capture=cvCreateFileCapture(filename);

    while(1)
    {
        img=cvqueryframe(capture) 

        if !img break;
        ///process img
    }

    cvreleasecapture(&capture);
}

Is there a way to fix the problem for multithreading? I was thinking of using

while(!capture) 
    capture=cvCreateFileCapture(filename);

but there should a more efficient way, maybe using locking Monitor::Enter(obj) or lock(obj)?