Forwarding this bug from Ubuntu reporter Lev Makhlis:
http://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/345224

[Problem]
Xorg freezes with 100% CPU within a minute of login.  Regression first appeared in Intrepid but is confirmed to still occur in Karmic with a recent -ati git snapshot.

[Original Description]
X started freezing once in a while after upgrading to Ubuntu 8.10.  Now with 9.04 it's even more common.
Mouse pointer works, keyboard doesn't work, can ssh remotely. Attaching backtrace.

Architecture: amd64
DistroRelease: Ubuntu 9.04
Package: xserver-xorg-video-ati 1:6.12.0-0ubuntu2
ProcEnviron:
 LANG=en_US.UTF-8ProcVersion: Linux version 2.6.28-11-generic (buildd@yellow) (gcc version 4.3.3 (Ubuntu 4.3.3-5ubuntu4) ) #35-Ubuntu SMP Wed Mar 18 21:55:34 UTC 2009
SourcePackage: xserver-xorg-video-ati
Uname: Linux 2.6.28-11-generic x86_64


#0  0x00007f692368acd7 in ioctl () from /lib/libc.so.6
No symbol table info available.
#1  0x00007f69222cf343 in drmIoctl () from /usr/lib/libdrm.so.2
No symbol table info available.
#2  0x00007f69222cf646 in drmCommandNone () from /usr/lib/libdrm.so.2
No symbol table info available.
#3  0x00007f6921462d74 in RADEONDownloadFromScreenCP (
    pSrc=<value optimized out>, x=0, y=18, w=17, h=0, dst=0x9aa4ba0 "", 
    dst_pitch=68) at ../../src/radeon_exa_funcs.c:411
	oldhpass = <value optimized out>
	i = 20984
	hpass = 0
	scratch_pitch_offset = 12060552
	indirect = {idx = 28090448, start = 50331648, end = 28363024, 
  discard = 0}
	__head = <value optimized out>
	pScrn = (ScrnInfoPtr) 0x1aca050
	src = (uint8_t *) 0x7f690ebfa000 "\033\005"
	bpp = <value optimized out>
	datatype = 6
	src_pitch_offset = 11955556
	scratch_pitch = 128
	scratch_off = 0
	scratch = (drmBufPtr) 0x1b0cd30
#4  0x00007f6920ba3c49 in exaCopyDirty (migrate=0x7fff2dab0100, 
    pValidDst=0x9a28e20, pValidSrc=<value optimized out>, 
    transfer=0x7f69214627d0 <RADEONDownloadFromScreenCP>, 
    fallback_src=0x7f691a6f3000 "\231\231\231", fallback_dst=0x9aa4ba0 "", 
    fallback_srcpitch=128, fallback_dstpitch=68, fallback_index=1, 
    sync=0x7f6920b9de10 <exaWaitSync>) at ../../exa/exa_migration.c:208
	pPixmap = (PixmapPtr) 0x9aa4b60
	pExaPixmap = (ExaPixmapPrivPtr) 0x9a28de0
	damage = <value optimized out>
	CopyReg = {extents = {x1 = 0, y1 = 0, x2 = 17, y2 = 18}, data = 0x0}
	save_offscreen = 1
	save_pitch = 128
	pBox = (BoxPtr) 0x7fff2daaff40
	nbox = 0
	access_prepared = 0
	need_sync = 0
	__func__ = "exaCopyDirty"
	firsttime = 0
#5  0x00007f6920ba3fc0 in exaDoMoveOutPixmap (migrate=0x7fff2dab0100)
    at ../../exa/exa_migration.c:256
	pPixmap = (PixmapPtr) 0x9aa4b60
#6  0x00007f6920ba46df in exaDoMigration (pixmaps=0x7fff2dab0100, npixmaps=1, 
    can_accel=0) at ../../exa/exa_migration.c:677
	pExaScr = (ExaScreenPrivPtr) 0x1b0c910
	i = 1
	j = <value optimized out>
	__func__ = "exaDoMigration"


[lspci]
00:00.0 Memory controller [0580]: nVidia Corporation CK804 Memory Controller [10de:005e] (rev a3)
    	Subsystem: ABIT Computer Corp. Device [147b:1c0c]
01:00.0 VGA compatible controller [0300]: ATI Technologies Inc RV380 [Radeon X600 (PCIE)] [1002:5b62]
    	Subsystem: ATI Technologies Inc Device [1002:0422]