Created attachment 76174 [details]
Xorg.log while X is using 100% CPU.

Xorg started using 100% CPU once, when I attempted to close a frozen application. The entire display became frozen except for the mouse pointer, which was moving normally, even changing shape, as if X was operating normally. I got two backtraces in this state:

#0  0xb7744424 in __kernel_vsyscall ()
#1  0xb7342f79 in ioctl () from /lib/libc.so.6
#2  0xb7128254 in drmIoctl () from /usr/lib/libdrm.so.2
#3  0xb712ae41 in drmCommandNone () from /usr/lib/libdrm.so.2
#4  0xb6d26f16 in ?? () from /usr/lib/xorg/modules/drivers/intel_drv.so
#5  0xb6d23774 in ?? () from /usr/lib/xorg/modules/drivers/intel_drv.so
#6  0x0814144a in AnimCurScreenBlockHandler (screenNum=0, blockData=0x0, pTimeout=0xbfd0ce74, 
    pReadmask=0x823a4a0 <LastSelectMask>) at animcur.c:186
#7  0x0807f5c6 in BlockHandler (pTimeout=pTimeout@entry=0xbfd0ce74, pReadmask=pReadmask@entry=0x823a4a0 <LastSelectMask>)
    at dixutils.c:387
#8  0x081b9b9a in WaitForSomething (pClientsReady=pClientsReady@entry=0x9fc9338) at WaitFor.c:210
#9  0x0807b34e in Dispatch () at dispatch.c:357
#10 0x08069165 in main (argc=7, argv=0xbfd0d1f4, envp=0xbfd0d214) at main.c:288

and

#0  0xb7744424 in __kernel_vsyscall ()
#1  0xb734456d in ___newselect_nocancel () from /lib/libc.so.6
#2  0x081b9c1c in WaitForSomething (pClientsReady=pClientsReady@entry=0x9fc9338) at WaitFor.c:221
#3  0x0807b34e in Dispatch () at dispatch.c:357
#4  0x08069165 in main (argc=7, argv=0xbfd0d1f4, envp=0xbfd0d214) at main.c:288

Also, twice so far X froze in WaitForSomething when I attempted to unlock the screen, after the plasma screenlocker locked it, although with normal CPU use. 

The bug report at https://bugzilla.redhat.com/show_bug.cgi?id=837144 seems to be related to this problem, and a patch can be found at http://lists.x.org/archives/xorg-devel/2011-October/026022.html, that allegedly fixes this.

This is with X.Org X Server 1.12.3.