-current XFree86 CPU hogging

For quite a while in -current, I've been hitting the situation where X
suddenly takes all the CPU and becomes unresponsive. I have to ssh into
the machine and kill XFree86. Ktracing it, I see:

   651 XFree86  RET   ioctl -1 errno 16 Device busy
   651 XFree86  PSIG  SIGALRM caught handler=0x80d4318 mask=0x0 code=0x0
   651 XFree86  CALL  sigreturn(0xbfbfe340)
   651 XFree86  RET   sigreturn JUSTRETURN
   651 XFree86  CALL  ioctl(0x7,0xc0286429 ,0xbfbfe680)
   651 XFree86  RET   ioctl -1 errno 16 Device busy
   651 XFree86  PSIG  SIGALRM caught handler=0x80d4318 mask=0x0 code=0x0
   651 XFree86  CALL  sigreturn(0xbfbfe340)
   651 XFree86  RET   sigreturn JUSTRETURN
   651 XFree86  CALL  ioctl(0x7,0xc0286429 ,0xbfbfe680)
   651 XFree86  RET   ioctl -1 errno 16 Device busy

This seems similar to the problem indicated in this thread:

http://marc.theaimsgroup.com/?l=freebsd-current&m=103401443626417&w=2

but not quite. It doesn't seem to correspond to any particular activity.
This is the only machine I get it on, and I've tried multiple upgrades
and rebuilding X and such. Any ideas would be much appreciated.

Kernel config:

http://redundancy.org/DEEPTHOUGHT

Dmesg:

http://redundancy.org/dmesg.txt