Bug 18895

Several reporters are finding the Backingstore option causes crashes on a range of different hardware:
https://bugs.edge.launchpad.net/ubuntu/+source/xorg-server/+bug/274988

Below is a backtrace from the crash.  Is this option supposed to work reliably?  If not, perhaps it should be deprecated?

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fe72afc46e0 (LWP 11393)]
0x00000000004de71e in miValidateTree (pParent=0x2144a40, 
    pChild=0x7fff32fe79c0, kind=VTMap) at ../../mi/mivaltree.c:790
	in ../../mi/mivaltree.c
(gdb) backtrace full
#0  0x00000000004de71e in miValidateTree (pParent=0x2144a40, 
    pChild=0x7fff32fe79c0, kind=VTMap) at ../../mi/mivaltree.c:790
	totalClip = {extents = {x1 = 0, y1 = 24, x2 = 0, y2 = 24}, 
  data = 0x7e1c60}
	childClip = {extents = {x1 = 0, y1 = 0, x2 = 0, y2 = 0}, 
  data = 0x7e1c60}
	childUnion = {extents = {x1 = 1, y1 = 0, x2 = 0, y2 = 0}, data = 0x0}
	exposed = {extents = {x1 = 0, y1 = 0, x2 = 0, y2 = 0}, 
  data = 0x7e1c60}
	pScreen = (ScreenPtr) 0x20e4a90
	pWin = (WindowPtr) 0x0
	overlap = 1
	viewvals = 0
	forward = 855538080
#1  0x00007fe728693813 in DRIValidateTree (pParent=0x2144a40, 
    pChild=0x28daa00, kind=VTMap) at ../../../../hw/xfree86/dri/dri.c:2055
	pScreen = (ScreenPtr) 0x20e4a90
	returnValue = <value optimized out>
#2  0x0000000000438e67 in MapWindow (pWin=0x28daa00, 
    client=<value optimized out>) at ../../dix/window.c:2811
	event = {u = {u = {type = 19 '\023', detail = 123 '{', 
      sequenceNumber = 1236}, keyButtonPointer = {pad00 = 81034003, 
      time = 104, root = 52429243, event = 1, child = 36695712, rootX = 0, 
      rootY = 0, eventX = 22480, eventY = 126, state = 0, 
      sameScreen = 0 '\0', pad1 = 0 '\0'}, enterLeave = {pad00 = 81034003, 
      time = 104, root = 52429243, event = 1, child = 36695712, rootX = 0, 
      rootY = 0, eventX = 22480, eventY = 126, state = 0, mode = 0 '\0', 
      flags = 0 '\0'}, focus = {pad00 = 81034003, window = 104, 
      mode = 187 '»', pad1 = 1 '\001', pad2 = 32 ' ', pad3 = 3 '\003'}, 
    expose = {pad00 = 81034003, window = 104, x = 443, y = 800, width = 1, 
      height = 0, count = 61088, pad2 = 559}, graphicsExposure = {
      pad00 = 81034003, drawable = 104, x = 443, y = 800, width = 1, 
      height = 0, minorEvent = 61088, count = 559, majorEvent = 0 '\0', 
      pad1 = 0 '\0', pad2 = 0 '\0', pad3 = 0 '\0'}, noExposure = {
      pad00 = 81034003, drawable = 104, minorEvent = 443, 
      majorEvent = 32 ' ', bpad = 3 '\003'}, visibility = {pad00 = 81034003, 
      window = 104, state = 187 '»', pad1 = 1 '\001', pad2 = 32 ' ', 
      pad3 = 3 '\003'}, createNotify = {pad00 = 81034003, parent = 104, 
      window = 52429243, x = 1, y = 0, width = 61088, height = 559, 
      borderWidth = 0, override = 0 '\0', bpad = 0 '\0'}, destroyNotify = {
      pad00 = 81034003, event = 104, window = 52429243}, unmapNotify = {
      pad00 = 81034003, event = 104, window = 52429243, 
      fromConfigure = 1 '\001', pad1 = 0 '\0', pad2 = 0 '\0', pad3 = 0 '\0'}, 
    mapNotify = {pad00 = 81034003, event = 104, window = 52429243, 
      override = 1 '\001', pad1 = 0 '\0', pad2 = 0 '\0', pad3 = 0 '\0'}, 
    mapRequest = {pad00 = 81034003, parent = 104, window = 52429243}, 
    reparent = {pad00 = 81034003, event = 104, window = 52429243, parent = 1, 
      x = -4448, y = 559, override = 0 '\0', pad1 = 0 '\0', pad2 = 0 '\0', 
      pad3 = 0 '\0'}, configureNotify = {pad00 = 81034003, event = 104, 
      window = 52429243, aboveSibling = 1, x = -4448, y = 559, width = 0, 
      height = 0, borderWidth = 22480, override = 126 '~', bpad = 0 '\0'}, 
    configureRequest = {pad00 = 81034003, parent = 104, window = 52429243, 
      sibling = 1, x = -4448, y = 559, width = 0, height = 0, 
      borderWidth = 22480, valueMask = 126, pad1 = 0}, gravity = {
      pad00 = 81034003, event = 104, window = 52429243, x = 1, y = 0, 
      pad1 = 36695712, pad2 = 0, pad3 = 8280016, pad4 = 0}, resizeRequest = {
      pad00 = 81034003, window = 104, width = 443, height = 800}, 
    circulate = {pad00 = 81034003, event = 104, window = 52429243, 
      parent = 1, place = 160 ' ', pad1 = 238 'î', pad2 = 47 '/', 
      pad3 = 2 '\002'}, property = {pad00 = 81034003, window = 104, 
      atom = 52429243, time = 1, state = 160 ' ', pad1 = 238 'î', 
      pad2 = 559}, selectionClear = {pad00 = 81034003, time = 104, 
      window = 52429243, atom = 1}, selectionRequest = {pad00 = 81034003, 
      time = 104, owner = 52429243, requestor = 1, selection = 36695712, 
      target = 0, property = 8280016}, selectionNotify = {pad00 = 81034003, 
      time = 104, requestor = 52429243, selection = 1, target = 36695712, 
      property = 0}, colormap = {pad00 = 81034003, window = 104, 
      colormap = 52429243, new = 1 '\001', state = 0 '\0', pad1 = 0 '\0', 
      pad2 = 0 '\0'}, mappingNotify = {pad00 = 81034003, request = 104 'h', 
      firstKeyCode = 0 '\0', count = 0 '\0', pad1 = 0 '\0'}, clientMessage = {
      pad00 = 81034003, window = 104, u = {l = {type = 52429243, longs0 = 1, 
          longs1 = 36695712, longs2 = 0, longs3 = 8280016, longs4 = 0}, s = {
          type = 52429243, shorts0 = 1, shorts1 = 0, shorts2 = -4448, 
          shorts3 = 559, shorts4 = 0, shorts5 = 0, shorts6 = 22480, 
          shorts7 = 126, shorts8 = 0, shorts9 = 0}, b = {type = 52429243, 
          bytes = "\001\000\000\000 î/\002\000\000\000\000ÐW~\000\000\000\000"}}}}}
	anyMarked = 1
	win_owner = (ClientPtr) 0x28bba10
	ag_leader = (ClientPtr) 0x0
	pScreen = (ScreenPtr) 0x20e4a90
	pParent = <value optimized out>
	dosave = 0
	pLayerWin = (WindowPtr) 0x28daa00
#3  0x000000000044cf66 in ProcMapWindow (client=0x28bba10)
    at ../../dix/dispatch.c:695
	pWin = (WindowPtr) 0x28daa00
	rc = 0
#4  0x000000000044d7a4 in Dispatch () at ../../dix/dispatch.c:454
	result = <value optimized out>
	client = (ClientPtr) 0x28bba10
	nready = 0
	start_tick = 800
#5  0x000000000043378d in main (argc=10, argv=0x7fff32fe7ca8, 
    envp=<value optimized out>) at ../../dix/main.c:441
	i = 1
	error = 0
	xauthfile = <value optimized out>
	alwaysCheckForInput = {0, 1}
(gdb) quit
The program is running.  Quit anyway (and detach it)? (y or n) Detaching from program: /usr/bin/Xorg, process 11393