Comment 3 for bug 91077

OK, it looks like assertion is being caused by the legacy NVIDIA GL library:

(gdb) thread apply all bt

Thread 1 (process 5358):
#0 0xb7f30410 in __kernel_vsyscall ()
#1 0xb7d92df0 in raise () from /lib/tls/i686/cmov/libc.so.6
#2 0xb7d94641 in abort () from /lib/tls/i686/cmov/libc.so.6
#3 0xb7d8c43b in __assert_fail () from /lib/tls/i686/cmov/libc.so.6
#4 0xb74ef6f4 in xcb_xlib_lock () from /usr/lib/libxcb-xlib.so.0
#5 0xb7cc0c8c in _XCBLockDisplay (dpy=0x804cb30) at ../../src/xcb_lock.c:20
#6 0xb7ca4241 in XESetCloseDisplay (dpy=0x804c340, extension=1, proc=0)
    at ../../src/InitExt.c:231
#7 0xb7edaa69 in ?? () from /usr/lib/libGL.so.1
#8 0x0804c340 in ?? ()
#9 0x00000001 in ?? ()
#10 0x00000000 in ?? ()

Using
LD_PRELOAD=/usr/lib/nvidia/libGL.so.1.2.xlibmesa glxgears
(thus using the original mesa GL library) makes the assertion part of this problem disappear.