[Mesa-dev] [Bug 99781] Some Unity games fail assertion on startup in glXCreateContextAttribsARB

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=99781

            Bug ID: 99781
           Summary: Some Unity games fail assertion on startup in
                    glXCreateContextAttribsARB
           Product: Mesa
           Version: 13.0
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: GLX
          Assignee: mesa-dev at lists.freedesktop.org
          Reporter: nmiell at gmail.com
        QA Contact: mesa-dev at lists.freedesktop.org

Multiple Unity games die during startup due to an assertion failure in
glXCreateContextAttribsARB.

The assertion in question is "Unknown sequence number while processing queue"
in poll_for_event() in libX11's src/xcb_io.c. I'm blaming Mesa for now because
you've got to start somewhere and some of the other potential culprits (Xlib,
XCB) also live in this Bugzilla.

Affected games include: Oxenfree, The Fall.

The stack trace looks like:
#0  __GI_raise (sig=sig at entry=6) at ../sysdeps/unix/sysv/linux/raise.c:58
#1  0x00007ffff61b051a in __GI_abort () at abort.c:89
#2  0x00007ffff61a6da7 in __assert_fail_base (fmt=<optimized out>,
assertion=assertion at entry=0x7ffff72b0778 "!xcb_xlib_threads_sequence_lost",
file=file at entry=0x7ffff72b05eb "xcb_io.c", 
    line=line at entry=259, function=function at entry=0x7ffff72b0a28
<__PRETTY_FUNCTION__.14787> "poll_for_event") at assert.c:92
#3  0x00007ffff61a6e52 in __GI___assert_fail
(assertion=assertion at entry=0x7ffff72b0778 "!xcb_xlib_threads_sequence_lost",
file=file at entry=0x7ffff72b05eb "xcb_io.c", line=line at entry=259, 
    function=function at entry=0x7ffff72b0a28 <__PRETTY_FUNCTION__.14787>
"poll_for_event") at assert.c:101
#4  0x00007ffff723e32a in poll_for_event (dpy=dpy at entry=0x21ca160) at
xcb_io.c:256
#5  0x00007ffff723e3db in poll_for_response (dpy=dpy at entry=0x21ca160) at
xcb_io.c:274
#6  0x00007ffff723e6cd in _XEventsQueued (dpy=0x21ca160, mode=<optimized out>)
at xcb_io.c:349
#7  0x00007ffff7241515 in _XGetRequest (dpy=0x21ca160, type=<optimized out>,
len=4) at XlibInt.c:1707
#8  0x00007ffff724162f in _XSeqSyncFunction (dpy=0x21ca160) at XlibInt.c:202
#9  0x00007ffff7240df3 in _XError (dpy=dpy at entry=0x21ca160,
rep=rep at entry=0x7fffffffd3e0) at XlibInt.c:1436
#10 0x00007ffff75591a2 in __glXSendErrorForXcb (dpy=dpy at entry=0x21ca160,
err=err at entry=0x304bd30) at glx_error.c:81
#11 0x00007ffff7555284 in glXCreateContextAttribsARB (dpy=0x21ca160,
config=0x22b3a20, share_context=0x220c3c0, direct=<optimized out>,
attrib_list=0x7fffffffd4a0) at create_context.c:119
#12 0x0000000000eac13f in ?? ()
#13 0x0000000000eace22 in ?? ()
#14 0x00000000004654a4 in ?? ()
#15 0x00007ffff6199401 in __libc_start_main (main=0x4641e0, argc=1,
argv=0x7fffffffdde8, init=<optimized out>, fini=<optimized out>,
rtld_fini=<optimized out>, stack_end=0x7fffffffddd8)
    at ../csu/libc-start.c:289

This appears to be timing-dependent or a race condition because the games will
sometimes successfully start (e.g. the first time run after booting or after
dropping the entire VFS cache), and will definitely start if you've set a
breakpoint somewhere that triggers at least once per call to
glXCreateContextAttribsARB.

As best as I can tell (by setting 4000 breakpoints in gdb using rbreak), Unity
does correctly call XInitThreads() before using Xlib or GLX or creating any
threads and only ever makes Xlib & GLX calls on thread 1 anyway. Which suggests
the race is between the game client and the X server, assuming it is a race.
This makes me wonder about the 32/64 sequence number widening in Xlib/XCB which
I do not understand at all. When the assertion fails, event_sequence is
something like 4294967378 (i.e. 2^32 + 82) while request is 84.

I've written a little LD_PRELOAD shim that calls usleep() in
glXCreateContextAttribsARB which makes the games completely playable.

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