I attempted to implement a readback of a drawn framebuffer for auto-exposure, but no matter what I try, either glGetTexImage or glMapBuffer stalls.
Am I doing this wrong, or this this an issue with my particular machine, because I can’t observe the stalling on my notebook.

struct ExposureReadback {
    int buffers = 3;
    int counter = 0;
    std::vector<DownloadPbo> pbos;

    ExposureReadback () {
        for (int i=0; i<buffers; ++i) {
            pbos.push_back( DownloadPbo{prints("ExposureReadback[%d]", i)} );
        }
        counter = 0;
    }

    bool readback (Render_Texture& tex, int2 full_res, lrgb* weighted_average) {
        int cur_buf = counter;
        counter = wrap(counter+1, buffers);
        int oldest_buf = counter;

        //...

        { // Trigger read into current pbo
            glBindBuffer(GL_PIXEL_PACK_BUFFER, pbos[cur_buf]);
            glBufferData(GL_PIXEL_PACK_BUFFER, size, nullptr, GL_STREAM_DRAW);
        
            glBindTexture(GL_TEXTURE_2D, tex);
            glGetTexImage(GL_TEXTURE_2D, mip, GL_RGB, GL_FLOAT, nullptr);
            glBindTexture(GL_TEXTURE_2D, 0);
        }
        
        bool readback_avail = false;
        { // Map now available oldest pbo
            glBindBuffer(GL_PIXEL_PACK_BUFFER, pbos[oldest_buf]);

            auto* mapped = (lrgb*)glMapBuffer(GL_PIXEL_PACK_BUFFER, GL_READ_ONLY);
            if (mapped) { // returns null for first few frames
                //...
                readback_avail = true;
        
                glUnmapBuffer(GL_PIXEL_PACK_BUFFER);
                glInvalidateBufferData(pbos[oldest_buf]);
            }
        }
        
        glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);

        return readback_avail;
    }
};

Profiler Results: