5.3-Beta7/RELENG_5, AGP/drm hard lock on xserver termination

Since installing RELENG_5 (marked as 5.3-Beta7) I've
experienced hard locks on xserver termination and more
generally when switching between the xserver and the other
text-mode ttys.  This happens with GENERIC + a custom kernel
with SMP removed.

I built a kernel with the debugger, witness, and invariants
only to discover that what had been very easy to reproduce
simply went away.  I suspect I've just made it much less
probable... so I'll go at it some more tonight after work.

AGP/drm locking suspected culprit? Can locking mistakes
create problems on UP systems? Because of kernel thread
preemption?

I have a vague sense I've read about a similar issue on this
list before, so if this is known problem, let me know so I
don't waste my time.

Thanks.