Sorry, but it is rather hard to provide you with some way to definitely be able to reproduce it. But at least here using kwin (kde 4.3.x atm) I do see those issues. Nothing that stops you from normal working, just some glitches. Those only happen with KMS active and it makes no difference if acceleration method for desktop effects is xrender or opengl. So I think the "fault" is somewhere in the KMS part, not in mesa or libdrm itself, though I am not sure at all.
Currently I am running 2.6.33-rc5, but I had the same observation with KMS in 2.6.32*. Using mesa, libdrm and xf86-video-radon from git-master.
Yeah, I have the same problem.
It shows up on and off. In the past, it was a Mesa commit that caused it, then it was reverted and things were OK. But it's there again.
I personally think KMS has quite a way to go on catching up with UMS in performance. As I have hacked away on both sets of code to improve performance by any means I just don't see where KMS is going to get it from or where the core bottle neck is. Minor improvements are possible however.
Note that there are some EXA bugs in xserver 1.7 that can cause these sorts of problems.
bug in kms radeon
in 2.6.33-rc6 not fixed
kernel full freeze and system not ping from remote computer
mesa from git and xf86-video-ati from git
system full freeze and not ping from remote computer