Bug: Deadlock for multi-threaded glx apps inside __glXInitialize()

[Mario Kleiner]

Hi Kristian

Testing with current mesa master, my toolkit deadlocks on the first  
call to a glX function (glXChooseVisual()).

The deadlock was probably introduced by your recent commit:

"glx: Use _Xglobal_lock for protecting extension display list"

ab434f6b7641a64d30725a9ac24929240362d466

The problem is that the _Xglobal_lock is locked twice inside the  
__glXInitialize() function of mesa/src/glx/glxext.c, once inside  
__glXInitialize(), and then as part of dri2CreateDisplay() -> ... ->  
XextFindDisplay. The 2nd locking call on the already held lock  
deadlocks.

Attached a backtrace with the problem and a patch/hack that "fixes"  
it for me, but introduces a race-condition itself, so this is  
obviously not the correct solution. The race condition would trigger  
if two threads would simultaneously do their first call to a glX  
function for the same Display* dpy handle, rather unlikely to happen  
in practice? If so, then the patch might be  an acceptable fix until  
a better solution is found?

Other applications (glxgears, games etc.) work correctly. The  
difference is that my toolkit calls  XInitThreads() at startup to use  
xlib multi-threaded and afaik the locking calls only get enabled if  
xlib is switched to thread-safe mode, otherwise they are no-ops?

thanks,
-mario

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Mario Kleiner
Max Planck Institute for Biological Cybernetics
Spemannstr. 38
72076 Tuebingen
Germany

e-mail: mario.kleiner at tuebingen.mpg.de
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