Re: [AD] XOR drawing under X11 (and what is _xwin_in_gfx_call)

[Chris <chris.kcat@xxxxxxxxxx>]

Elias Pschernig wrote:
> Hm. I think it can't work. Just think of cases like this:
>
> thread 1:
> XLOCK();
> bmp->vtable = &orig_vtable;
> *** context switch ***
>
> thread 2:
> draw(bmp); /* will use the wrong vtable method */

In that case, the second thread's draw operation will block on XLOCK 
until the first thread completes drawing and XUNLOCKs. Dealing with the 
SIGALRM version may be problematic, but I think this could wait until 
4.3 when we drop it.

> Maybe we could copy the bitmap, like this:
>
> void _some_draw_function()
> {
>     BITMAP temp = *bmp;
>     for (i = h; i < bmp->h; i++) temp.line[i] = bmp->line[i];

Right there you'd be overwriting memory. Don't forget the line[] 
pointers reference memory that extend out past the structure.

> For Allegro 4.3.x, I think, we could require all access to a bitmap to
> be locked. That is, a function like putpixel would look like this:
>
> acquire_bitmap ();
> vtable->putpixel
> release_bitmap ()
>
> instead of right now:
>
> vtable->putpixel /* locking is done by the vtable method */

I thought about this, and here's the thing. Granted putpixel is a poor 
example here, but if a vtable entry can potentially be hardware 
accelerated, you need the surface unlocked to do it. Forcing a lock only 
to have it unlocked by the vtable entry before the operation even takes 
place is a bit wasteful, don't you think?