Re: xsrc/58133: X server crashes; radeon 5450; modesetting

[Rhialto <rhialto%falu.nl@localhost>]

I noticed that in the cases where I could check, vbo_offset == 0.
That means that the flow through glamor_get_vbo_space() must have gone
like so:

glamor_get_vbo_space(ScreenPtr screen, unsigned size, char **vbo_offset)
{
    glamor_screen_private *glamor_priv = glamor_get_screen_private(screen);
    void *data;

    glamor_make_current(glamor_priv);

    glBindBuffer(GL_ARRAY_BUFFER, glamor_priv->vbo);

    if (glamor_priv->has_buffer_storage) {	// YES
      if (glamor_priv->vbo_size < glamor_priv->vbo_offset + size) {
           if (glamor_priv->vbo_size)
                glUnmapBuffer(GL_ARRAY_BUFFER);

            if (size > glamor_priv->vbo_size) { // NO
		// allocate a bigger buffer,
		// glamor_priv->vbo_size = MAX(GLAMOR_VBO_SIZE, size);
            }

            glamor_priv->vbo_offset = 0;
            glamor_priv->vb = glMapBufferRange(GL_ARRAY_BUFFER,
                                               0, glamor_priv->vbo_size,
                                               GL_MAP_WRITE_BIT |
                                               GL_MAP_INVALIDATE_BUFFER_BIT |
                                               GL_MAP_PERSISTENT_BIT |
                                               GL_MAP_COHERENT_BIT);
        }
        *vbo_offset = (void *)(uintptr_t)glamor_priv->vbo_offset;
        data = glamor_priv->vb + glamor_priv->vbo_offset;
        glamor_priv->vbo_offset += size;
    } else // IRRELEVANT
    return data;
}

This is consistent with the case where the crash was in
glamor_composite_glyphs(), where `glamor_screen_private *glamor_priv` is
available, and I could check for has_buffer_storage etc.

I couldn't help wondering if glMapBufferRange() (documented at
https://registry.khronos.org/OpenGL-Refpages/gl4/html/glMapBufferRange.xhtml
I think) might perhaps work (slightly) async, so that by the time
`*data` is written to, the mapping is not yet complete. gdb had no
trouble to read v[0] in the core dump.

Also I wonder why GL_MAP_READ_BIT is not included. I only see writing to
the buffer, but who trusts the compiler not to read from it...

I also wonder why the unmapping and remapping of the buffer isn't moved
into the inner condition (the one which is false for our case, where a
bigger buffer is allocated). That ought to be more efficient.

It is unlikely that this is the first time through
glamor_get_vbo_space(); many characters have been printed before.

In case it matters, in most of my xterms I use bitmap fonts,
lucidasanstypewriter-12, which is as far as I can tell indeed a 7x13
font (width and height as seen in `info locals` before).