(In reply to comment #29)
> Created attachment 86051 [details] [review] [review]
> DRI2: Install client callback only once
> 
> Does this patch fix the problem?

This, plus the patch in comment 6, fixed it for me (Lubuntu saucy, kernel
3.12-rc1 amd64, xserver-xorg-video-radeon 1:7.2.0-0ubuntu6 ).
Thanks people, thank you very much!