kern/46625: 6.0_BETA2: Xorg is in drmwtq/0 state -> system freeze

[cheusov%tut.by@localhost]

>Number:         46625
>Category:       kern
>Synopsis:       6.0_BETA2: Xorg is in drmwtq/0 state -> system freeze
>Confidential:   no
>Severity:       critical
>Priority:       high
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Jun 23 12:00:01 +0000 2012
>Originator:     Aleksey Cheusov
>Release:        NetBSD 6.0_BETA2 i386
>Organization:
none
>Environment:
NetBSD work.imb.invention.com 6.0_BETA2 NetBSD 6.0_BETA2 (GENERIC) #11: Sat Jun 
16 17:12:51 FET 2012  
cheusov%work.imb.invention.com@localhost:/srv/obj-current/sys/arch/i386/compile/GENERIC
 i386
>Description:
See below
>How-To-Repeat:
netbsd-6 + difference between revisions 1.10 and 1.11
of sys/external/bsd/drm/dist/bsd-core/drm_bufs.c (See PR 46208 for details).

1) /etc/rc.d/xdm start
2) press Ctrl-Alt-F2 for switching to the console
3) press Ctrl-Alt-F5 for switching to xdm again.
After this xdm/Xorg hangs up.
That is, the system stays in graphical mode, xdm is not reappeared completely,
mouse and keyboard do not work, so,
I cannot login and Ctrl-Alt-Fn doesn't work too.
The system is usable, though, I'm able to ssh to it remotely and reboot.

'top' says that Xorg is in drmwtq/0 state. gdb's backtrace:

#0  0xbb8eee67 in ioctl () from /usr/lib/libc.so.12
#1  0xbbaa507b in drmIoctl () from /usr/X11R7/lib/libdrm.so.3
#2  0xbbaa7597 in drmCommandWrite () from /usr/X11R7/lib/libdrm.so.3
#3  0xbb47b748 in I830Sync () from /usr/X11R7/lib/modules/drivers/intel_drv.so
#4  0xbb449cc3 in ?? () from /usr/X11R7/lib/modules/drivers/intel_drv.so
#5  0xbb3360e8 in exaWaitSync () from /usr/X11R7/lib/modules/libexa.so
#6  0xbb336324 in ExaDoPrepareAccess () from /usr/X11R7/lib/modules/libexa.so
#7  0xbb33393b in ?? () from /usr/X11R7/lib/modules/libexa.so
#8  0xbb333b34 in exaCopyDirtyToFb () from /usr/X11R7/lib/modules/libexa.so
#9  0xbb333c8c in ?? () from /usr/X11R7/lib/modules/libexa.so
#10 0xbb3343d7 in exaDoMigration_classic () from
/usr/X11R7/lib/modules/libexa.so
#11 0xbb33614f in exaDoMigration () from /usr/X11R7/lib/modules/libexa.so
#12 0xbb32afc6 in ?? () from /usr/X11R7/lib/modules/libexa.so
#13 0xbb32b51a in exaCompositeRects () from /usr/X11R7/lib/modules/libexa.so
#14 0xbb32efb5 in exaGlyphs () from /usr/X11R7/lib/modules/libexa.so
#15 0x08149e74 in ?? ()
#16 0x08132924 in ?? ()
#17 0x080952a2 in Dispatch ()
#18 0x0806f1aa in main ()

>Fix: