This is a bug report regarding not calling XInitThreads(). Especially if underlying OpenGL implementation uses threads. It's copied from a Fedora bug report <https://bugzilla.redhat.com/show_bug.cgi?id=1181085>:

Some libGL implementations call XInitThreads() on dlopen. For example, the
latest nVidia proprietary implementation does it, probably because it needs to
use threads for performance. This causes applications that use SDL 1.2 to crash. 

From XInitThreads(3):

	The XInitThreads function initializes Xlib support for concurrent threads.
	This function must be the first Xlib function a multi-threaded program
	calls, and it must complete before any other Xlib call is made.

From <https://wiki.libsdl.org/SDL_GL_LoadLibrary>:

	This should be done after initializing the video driver, but before creating
	any OpenGL windows. If no OpenGL library is loaded, the default library will
	be loaded upon creation of the first OpenGL window.

But SDL_VideoInit() calls XOpenDisplay() and nVidia libGL implementation calls
XInitThreads() on dlopen:

	(gdb) break XInitThreads
	Function "XInitThreads" not defined.
	Make breakpoint pending on future shared library load? (y or [n]) y
	Breakpoint 1 (XInitThreads) pending.
	(gdb) run
	Starting program: /mnt/data/home/woky/src/xonotic/darkplaces/darkplaces-sdl 
	[Thread debugging using libthread_db enabled]
	Using host libthread_db library "/lib64/libthread_db.so.1".
	Game is DarkPlaces-Quake using base gamedir id1
	gamename for server filtering: DarkPlaces-Quake
	DarkPlaces-Quake Linux 18:50:26 Jan 11 2015 - debug
	Current nice level is below the soft limit - cannot use niceness
	Playing shareware version.
	Skeletal animation uses SSE code path
	DPSOFTRAST available (SSE2 instructions detected)
	Failed to init SDL joystick subsystem: 
	couldn't exec quake.rc
	couldn't exec default.cfg
	execing config.cfg
	couldn't exec autoexec.cfg
	Client using an automatically assigned port
	Client opened a socket on address 0.0.0.0:0
	Linked against SDL version 1.2.15
	Using SDL library version 1.2.15

	Breakpoint 1, XInitThreads () at locking.c:573
	573	{
	(gdb) thread apply all bt

	Thread 1 (Thread 0x7ffff7fcc700 (LWP 8884)):
	#0  XInitThreads () at locking.c:573
	#1  0x00007fffea173136 in ?? () from /usr/lib64/nvidia/libGL.so.1
	#2  0x00007fffea1956cd in ?? () from /usr/lib64/nvidia/libGL.so.1
	#3  0x00007fffea173b9d in ?? () from /usr/lib64/nvidia/libGL.so.1
	#4  0x00007ffff7deaf0d in call_init (l=0x2923d80, argc=argc@entry=1, argv=argv@entry=0x7fffffffe038, env=env@entry=0x7fffffffe048) at dl-init.c:62
	#5  0x00007ffff7deb05b in call_init (env=<optimized out>, argv=<optimized out>, argc=<optimized out>, l=<optimized out>) at dl-init.c:34
	#6  _dl_init (main_map=main_map@entry=0x2923d80, argc=1, argv=0x7fffffffe038, env=0x7fffffffe048) at dl-init.c:124
	#7  0x00007ffff7defa51 in dl_open_worker (a=a@entry=0x7fffffffcdc8) at dl-open.c:566
	#8  0x00007ffff7deadf4 in _dl_catch_error (objname=objname@entry=0x7fffffffcdb8, errstring=errstring@entry=0x7fffffffcdc0, mallocedp=mallocedp@entry=0x7fffffffcdb7, 
			operate=operate@entry=0x7ffff7def590 <dl_open_worker>, args=args@entry=0x7fffffffcdc8) at dl-error.c:187
	#9  0x00007ffff7deeee3 in _dl_open (file=0x7ffff701a7b6 "libGL.so.1", mode=-2147483390, caller_dlopen=0x7ffff6ff968b <X11_GL_LoadLibrary+59>, nsid=-2, argc=<optimized out>, 
			argv=<optimized out>, env=0x7fffffffe048) at dl-open.c:650
	#10 0x00007ffff725c039 in dlopen_doit (a=a@entry=0x7fffffffcfe0) at dlopen.c:66
	#11 0x00007ffff7deadf4 in _dl_catch_error (objname=0x292d090, errstring=0x292d098, mallocedp=0x292d088, operate=0x7ffff725bfe0 <dlopen_doit>, args=0x7fffffffcfe0) at dl-error.c:187
	#12 0x00007ffff725c69d in _dlerror_run (operate=operate@entry=0x7ffff725bfe0 <dlopen_doit>, args=args@entry=0x7fffffffcfe0) at dlerror.c:163
	#13 0x00007ffff725c0d1 in __dlopen (file=file@entry=0x7ffff701a7b6 "libGL.so.1", mode=mode@entry=258) at dlopen.c:87
	#14 0x00007ffff6ff968b in X11_GL_LoadLibrary (this=0x2975760, path=0x7ffff701a7b6 "libGL.so.1") at src/video/x11/SDL_x11gl.c:506
	#15 0x0000000000409616 in VID_InitModeGL (mode=0x7fffffffd4e0) at ../../../vid_sdl.c:2461
	#16 0x0000000000409cdf in VID_InitMode (mode=0x7fffffffd4e0) at ../../../vid_sdl.c:2758
	#17 0x00000000006fdbfd in VID_Mode (fullscreen=1, width=640, height=480, bpp=32, refreshrate=60, stereobuffer=0, samples=1) at ../../../vid_shared.c:1857
	#18 0x00000000006fe4e5 in VID_Start () at ../../../vid_shared.c:2022
	#19 0x000000000057400c in Host_StartVideo () at ../../../host.c:1088
	#20 0x000000000047ad16 in SCR_BeginLoadingPlaque (startup=true) at ../../../cl_screen.c:761
	#21 0x000000000057454b in Host_Init () at ../../../host.c:1324
	#22 0x0000000000572d99 in Host_Main () at ../../../host.c:679
	#23 0x000000000040444e in main (argc=1, argv=0x7fffffffe038) at ../../../sys_sdl.c:223

So if a program is written according to the SDL docs and runs with nVidia libGL
implementation, the XInitThreads() is called too late from libGL and bad things
happen[1]. This problem doesn't occur with SDL 2.0.3 because it calls
XInitThreads() during video initialization.

I experience this problem for about 3-6 months now so it's probably caused by
change in nVidia proprietary libGL implementation. However, it breaks some
packages already present in Fedora which use SDL 1.2. Xonotic is affected for
example. The crash happens during closing of the display where libX11 tries to
release mutex which is NULL (or some random memory, see [1]).

SDL 1.2 should be patched to explicitly call XInitThreads in SDL_VideoInit() to
avoid such crashes under arbitrary libGL implementations.

I wouldn't be able to figure this out without the insight of Xonotic core team
wizards, especially divVerent. Thank you.

[1] http://forums.fedoraforum.org/showthread.php?t=302471

Version-Release number of selected component (if applicable): SDL-1.2.15-17.fc21.x86_64

Steps to Reproduce:
1. sudo dnf install xonotic
2. xonotic-sdl
3. # from game UI, either exit the game or change the resolution

Actual results: Game segfaults.

Expected results: Game doesn't segfault.