While using SNA architecture - I've found out, that leaving xlock running on the machine usually kills Xorg after some time.
I'm not exactly sure which xlock mode is 'killer' - it usually happens
when I leave machine for 1/2 hour and return back.

There seems to be no kernel GPU hung reported in kernel message 
only Xorg crash coredump.

sna git commit: 55e6f5f220401318529e81f7c96fe0af3b893a0c
xorg-x11-server-Xorg-1.12.1-2.fc18.x86_64
linux kernel  3.4   upstream.
hw: T61, GMA965, 4GB

#0  0x00007f61e7083c05 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:63
#1  0x00007f61e70853b8 in __GI_abort () at abort.c:90
#2  0x00007f61e707ca32 in __assert_fail_base (fmt=0x7f61e71c9c28 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", 
    assertion=assertion@entry=0x7f61e5472a90 "kgem->nexec < (sizeof(kgem->exec)/sizeof((kgem->exec)[0]))", 
    file=file@entry=0x7f61e54724e2 "kgem.c", line=line@entry=913, function=function@entry=0x7f61e5473400 "kgem_add_handle")
    at assert.c:93
#3  0x00007f61e707cae2 in __GI___assert_fail (
    assertion=assertion@entry=0x7f61e5472a90 "kgem->nexec < (sizeof(kgem->exec)/sizeof((kgem->exec)[0]))", 
    file=file@entry=0x7f61e54724e2 "kgem.c", line=line@entry=913, function=function@entry=0x7f61e5473400 "kgem_add_handle")
    at assert.c:102
#4  0x00007f61e53b8586 in kgem_add_handle (bo=<optimized out>, kgem=<optimized out>) at kgem.c:913
#5  _kgem_add_bo (kgem=kgem@entry=0x7f61e9131218, bo=<optimized out>) at kgem.c:925
#6  0x00007f61e53be4ba in kgem_add_reloc (kgem=kgem@entry=0x7f61e9131218, pos=<optimized out>, bo=<optimized out>, 
    bo@entry=0x1e6e320, read_write_domain=read_write_domain@entry=163840, delta=<optimized out>, delta@entry=0)
    at kgem.c:3119
#7  0x00007f61e53c2ed1 in sna_copy_bitmap_blt (_bitmap=_bitmap@entry=0x419a030, drawable=<optimized out>, gc=0x231f1d0, 
    box=box@entry=0x7fffa6a73980, n=<optimized out>, n@entry=1, sx=sx@entry=0, sy=sy@entry=0, reverse=reverse@entry=0, 
    upsidedown=0, bitplane=bitplane@entry=1, closure=0x7fffa6a73a30) at sna_accel.c:5253
#8  0x000000000054eab5 in miCopyRegion (pSrcDrawable=pSrcDrawable@entry=0x419a030, 
    pDstDrawable=pDstDrawable@entry=0x1ddc170, pGC=pGC@entry=0x231f1d0, pDstRegion=pDstRegion@entry=0x7fffa6a73980, 
    dx=dx@entry=0, dy=dy@entry=0, copyProc=copyProc@entry=0x7f61e53c2990 <sna_copy_bitmap_blt>, bitPlane=bitPlane@entry=1, 
    closure=closure@entry=0x7fffa6a73a30) at micopy.c:121
#9  0x000000000054f056 in miDoCopy (pSrcDrawable=pSrcDrawable@entry=0x419a030, pDstDrawable=pDstDrawable@entry=0x1ddc170, 
    pGC=pGC@entry=0x231f1d0, xIn=xIn@entry=0, yIn=yIn@entry=0, widthSrc=1680, heightSrc=heightSrc@entry=1050, 
    xOut=xOut@entry=0, yOut=yOut@entry=0, copyProc=0x7f61e53c2990 <sna_copy_bitmap_blt>, bitPlane=bitPlane@entry=1, 
    closure=closure@entry=0x7fffa6a73a30) at micopy.c:297
#10 0x00007f61e53da013 in sna_copy_plane (src=0x419a030, dst=0x1ddc170, gc=0x231f1d0, src_x=0, src_y=0, w=<optimized out>, 
    h=1050, dst_x=0, dst_y=0, bit=1) at sna_accel.c:5583
#11 0x0000000000504a0c in damageCopyPlane (pSrc=0x419a030, pDst=0x1ddc170, pGC=0x231f1d0, srcx=0, srcy=<optimized out>, 
    width=1680, height=1050, dstx=0, dsty=0, bitPlane=1) at damage.c:857
#12 0x0000000000430635 in ProcCopyPlane (client=0x2b6f2e0) at dispatch.c:1667
#13 0x000000000043442a in Dispatch () at dispatch.c:425
#14 0x0000000000423495 in main (argc=4, argv=0x7fffa6a73d28, envp=<optimized out>) at main.c:288