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

[bugzilla-daemon]

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

GitLab Migration User  changed:

   What|Removed |Added

 Status|REOPENED|RESOLVED
 Resolution|--- |MOVED

--- Comment #25 from GitLab Migration User  ---
-- GitLab Migration Automatic Message --

This bug has been migrated to freedesktop.org's GitLab instance and has been
closed from further activity.

You can subscribe and participate further through the new bug through this link
to our GitLab instance: https://gitlab.freedesktop.org/mesa/mesa/issues/108.

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

[bugzilla-daemon]

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

--- Comment #24 from Hal Gentz  ---
s/if some other thread locking/if some other thread locks/

Sorry, again.

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

[bugzilla-daemon]

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

--- Comment #23 from Hal Gentz  ---
s/blocks with/blocks from/
s/suspician/suspicion/
s/the mutex/the mutex is/

Apologies in advance for inconsistent back ticking and not spell checking. I
really wish I could edit replies.

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

[bugzilla-daemon]

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

--- Comment #22 from Hal Gentz  ---
Created attachment 144685
  --> https://bugs.freedesktop.org/attachment.cgi?id=144685=edit
Requested patch (does not work)

I tried implementing your suggested patch, Uli, but it did not work. 

XSync also tried to call LockDisplay, which for some reason blocks with our
call to LockDisplay. This behavior, I think, contradicts what the XLockDisplay
docs say: 

>  Nested calls to XLockDisplay work correctly; the display will not actually 
> be unlocked until XUnlockDisplay has been called the same number of times as 
> XLockDisplay.

This behavior really suprized me, so I'm not sure what's up. My only suspician
is that the mutex not being made with a call to pthread_mutex_init w/ the
PTHREAD_MUTEX_RECURSIVE attribute. I tried grepping for `pthread_mutex_init`,
`PTHREAD_MUTEX_RECURSIVE`, `pthread` and `RECURSIVE` in the libx11 codebase to
no avail. The only file I found was `UIThrStubs.c`, which if I understand
correctly, is only used when multi-threading support is disabled at compile
time.

I found a call to some `xmutex_init` function in locking.c, however, this
function only receives one parameter, unlike the two required by
`pthread_mutex_init`. gdb shows (I had to use `si` instead of `s`) that calls
to `xmutex_init` actually result in `pthread_mutex_init` getting called,
however, I'm not sure if that's because `xmutex_init` is some clever macro, or
if `-O3` inlined it. Nevertheless, I could not find any definition for
`xmutex_init` beyond the one in `UIThrStubs.c`.


I think we could possibly move the call to XSync after the call to
UnlockDisplay, however, I'm concerned that this might lead to some sort of race
condition if some other thread locking the display between our call to
UnlockDisplay and XSync's call to LockDisplay.

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

[bugzilla-daemon]

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

Hal Gentz  changed:

   What|Removed |Added

   Assignee|mesa-dev@lists.freedesktop. |zegen...@protonmail.com
   |org |

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

[bugzilla-daemon]

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

--- Comment #21 from Hal Gentz  ---
I just spotted that this got reopened today when going through my emails for
the first time in forever... this is unfortunate.

I'll take another swing at this sometime next week using Uli Schlachter's new
proposed method. Hopefully that doesn't cause a regression.

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

[bugzilla-daemon]

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

--- Comment #20 from Uli Schlachter  ---
Random guess for where the regression comes from:

X11DRV_expect_error() is used to say "I expect that the next request might
fail":
https://github.com/wine-mirror/wine/blob/6d801377055911d914226a3c6af8d8637a63fa13/dlls/winex11.drv/x11drv_main.c#L228-L241

...which is then used by the error handler to check if the error is expected
and should be ignored:
https://github.com/wine-mirror/wine/blob/6d801377055911d914226a3c6af8d8637a63fa13/dlls/winex11.drv/x11drv_main.c#L262-L271

So... apparently Wine wants to catch errors for "the next X11 request" while
mesa tries to invent errors that do not come from any X11 request.

Another random idea for a fix would be: Add a call to XSync(dpy, False) to the
"invent an error"-functions. That should guarantee that dpy->request ==
dpy->last_request_read and so it does not matter any more which of the two
sequence numbers mesa uses.

(Well, actually XSync() does a XGetInputFocus() internally, so mesa would then
use the sequence number of this GetInputFocus request for its claim that
something failed that never happened.)

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

[bugzilla-daemon]

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

Timothy Arceri  changed:

   What|Removed |Added

 Status|RESOLVED|REOPENED
 Resolution|FIXED   |---

--- Comment #19 from Timothy Arceri  ---
I've reverted this for now as it was causing regressions bug #110632 and bug
#110590

commit a01b393c397c846345f03f76f1167dd667e0ee96
Author: Timothy Arceri 
Date:   Tue May 7 13:55:32 2019 +1000

Revert "glx: Fix synthetic error generation in __glXSendError"

This reverts commit e91ee763c378d03883eb88cf0eadd8aa916f7878.

This seems to have broken a number of wine games. Lets revert
everything for now and try again later.

Acked-by: Adam Jackson 
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110632
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110590

Looks like we need better piglit tests before trying again.

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

[bugzilla-daemon]

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

Michel Dänzer  changed:

   What|Removed |Added

 Resolution|--- |FIXED
 Status|NEW |RESOLVED

--- Comment #18 from Michel Dänzer  ---
Thanks for the report, fixed in Mesa Git master:

https://gitlab.freedesktop.org/mesa/mesa/commit/e91ee763c378d03883eb88cf0eadd8aa916f7878

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

[bugzilla-daemon]

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

tele4...@hotmail.com changed:

   What|Removed |Added

 CC||tele4...@hotmail.com

--- Comment #17 from tele4...@hotmail.com ---
I happened across this issue with a Unity 5.3.8 game and Mesa 18.3.6 on a
Sandybridge / xf86-video-intel laptop. The patch linked in comment #15 fixed it
for me.

Please consider CC'ing mesa-stable with the fix.

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

[bugzilla-daemon]

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

--- Comment #16 from Ian Romanick  ---
(In reply to Hal Gentz from comment #10)
> Created attachment 143764 [details]
> Improved repo of the bug.

I would very much like to see this as a piglit test.

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

[bugzilla-daemon]

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

--- Comment #15 from Hal Gentz  ---
https://lists.freedesktop.org/archives/mesa-dev/2019-March/217153.html

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

[bugzilla-daemon]

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

--- Comment #14 from Hal Gentz  ---
Created attachment 143767
  --> https://bugs.freedesktop.org/attachment.cgi?id=143767=edit
Patch that fixes bug.

Hey,

This patch fixes this bug. I had to also patch `__glXSendErrorForXcb` because
`glXCreateContextAttribsARB` actually uses the former.

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

[bugzilla-daemon]

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

--- Comment #13 from Uli Schlachter  ---
Created attachment 143766
  --> https://bugs.freedesktop.org/attachment.cgi?id=143766=edit
A single-threaded reproducer

I think I came up with a single-threaded program reproducing the bug. More
details in inline comments.

When changing the copy of __glXSendError() from using dpy->request to
dpy->last_request_read (and compiling with -DNDEBUG so that the inline
asserts() do not trigger), the problem goes away. So, I think the analysis from
my previous comment is correct.

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

[bugzilla-daemon]

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

Hal Gentz  changed:

   What|Removed |Added

 Attachment #143763|0   |1
is obsolete||

--- Comment #10 from Hal Gentz  ---
Created attachment 143764
  --> https://bugs.freedesktop.org/attachment.cgi?id=143764=edit
Improved repo of the bug.

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

[bugzilla-daemon]

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

Uli Schlachter  changed:

   What|Removed |Added

 CC||psyc...@znc.in

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

[bugzilla-daemon]

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

--- Comment #12 from Uli Schlachter  ---
I took a look at this and I would guess that the bug is in mesa. However, I do
not want to built mesa myself and thus I will not test my theory.

The function __glXSendError() in mesa's src/glx/glx_error.c invents an X11
protocol error out of thin air. For the sequence number it uses dpy->request.
This is the sequence number of the last request that was sent. _XError() will
then update dpy->last_request_read based on the sequence number of the error
that just "came in".

If now another something comes in with a sequence number less than
dpy->last_request_read, since sequence numbers are monotonically increasing,
widen() will incorrectly add 1<<32 to the sequence number and things might go
downhill afterwards.

At least that's my theory. If my theory is right, changing mesa's
__glXSendError() to use dpy->last_request_read instead of dpy->request should
fix things.

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

[bugzilla-daemon]

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

Hal Gentz  changed:

   What|Removed |Added

 CC||zegen...@protonmail.com

--- Comment #8 from Hal Gentz  ---
Created attachment 143763
  --> https://bugs.freedesktop.org/attachment.cgi?id=143763=edit
Repo the bug

Some of the users of my project, glutin, stumbled upon this bug,
https://github.com/tomaka/glutin/issues/1034

While I don't yet have a solution, I got a small C program this (for my
computer at least) always reproduces the issue.

Run via `cc -o gl3 main.c -lGL -lX11 -lpthread -g && ./gl3`.

```
$ pacman -Q | grep -i
"mesa\|llvm\|clang\|glvnd\|vulkan\|amdgpu\|radeon\|libdrm\|amdvlk\|xorg\|x11\|xcb\|glx"
amdvlk-git r40.48d46fa-1
clang-svn 356068-1
clang-tools-extra-svn 356068-1
lib32-amdvlk-git r40.48d46fa-1
lib32-libdrm-git 6503.4735ca71-1
lib32-libglvnd 1.1.1-1
lib32-libx11 1.6.7-1
lib32-libxcb 1.13.1-1
lib32-llvm-libs-svn 356086-1
lib32-llvm-svn 356086-1
lib32-mesa-git 109188.03a0801bcb1-1
lib32-vulkan-icd-loader 1.1.101-1
lib32-vulkan-radeon-git 109188.03a0801bcb1-1
libdrm-git 6503.4735ca71-1
libdrm-git-debug 6503.4735ca71-1
libglvnd-git 1.1.0.r13.gf92208b-1
libx11 1.6.7-1
libxcb 1.13.1-1
libxkbcommon-x11 0.8.4-1
llvm-libs-svn 356068-1
llvm-ocaml-svn 356068-1
llvm-svn 356068-1
mesa-demos 8.4.0-1
mesa-git 109188.03a0801bcb1-1
mesa-git-debug 109188.03a0801bcb1-1
opencl-mesa-git 109188.03a0801bcb1-1
qt5-x11extras 5.12.2-1
vulkan-headers 1:1.1.102-2
vulkan-icd-loader 1.1.101-1
vulkan-radeon-git 109188.03a0801bcb1-1
vulkan-tools 1.1.101-1
vulkan-validation-layers 1.1.100-2
xcb-proto 1.13-2
xcb-util 0.4.0-2
xcb-util-cursor 0.1.3-2
xcb-util-image 0.4.0-2
xcb-util-keysyms 0.4.0-2
xcb-util-renderutil 0.3.9-2
xcb-util-wm 0.4.1-2
xcb-util-xrm 1.3-1
xf86-video-amdgpu-git 498.6ee8577-1
xf86-video-amdgpu-git-debug 498.6ee8577-1
xorg-bdftopcf 1.1-1
xorg-docs 1.7.1-2
xorg-font-util 1.3.1-2
xorg-font-utils 7.6-5
xorg-fonts-100dpi 1.0.3-4
xorg-fonts-75dpi 1.0.3-4
xorg-fonts-alias 1.0.3-2
xorg-fonts-encodings 1.0.4-5
xorg-iceauth 1.0.8-1
xorg-luit 1.1.1-3
xorg-mkfontscale 1.2.0-2
xorg-server 1.20.4-1
xorg-server-common 1.20.4-1
xorg-server-devel 1.20.4-1
xorg-server-xdmx 1.20.4-1
xorg-server-xephyr 1.20.4-1
xorg-server-xnest 1.20.4-1
xorg-server-xvfb 1.20.4-1
xorg-server-xwayland 1.20.4-1
xorg-sessreg 1.1.1-2
xorg-setxkbmap 1.3.1-2
xorg-smproxy 1.0.6-2
xorg-util-macros 1.19.2-1
xorg-x11perf 1.6.0-2
xorg-xauth 1.0.10-2
xorg-xbacklight 1.2.2-1
xorg-xclock 1.0.8-1
xorg-xcmsdb 1.0.5-2
xorg-xcursorgen 1.0.7-1
xorg-xdpyinfo 1.3.2-2
xorg-xdriinfo 1.0.6-1
xorg-xev 1.2.3-1
xorg-xfontsel 1.0.6-1
xorg-xgamma 1.0.6-2
xorg-xhost 1.0.8-1
xorg-xinit 1.4.1-1
xorg-xinput 1.6.2-2
xorg-xkbcomp 1.4.2-1
xorg-xkbevd 1.1.4-2
xorg-xkbutils 1.0.4-3
xorg-xkill 1.0.5-1
xorg-xlsatoms 1.1.3-1
xorg-xlsclients 1.1.4-1
xorg-xmessage 1.0.5-1
xorg-xmodmap 1.0.10-1
xorg-xpr 1.0.5-1
xorg-xprop 1.2.4-1
xorg-xrandr 1.5.0-2
xorg-xrdb 1.2.0-1
xorg-xrefresh 1.0.6-1
xorg-xset 1.2.4-1
xorg-xsetroot 1.1.2-1
xorg-xvinfo 1.1.4-1
xorg-xwd 1.0.7-1
xorg-xwininfo 1.1.4-1
xorg-xwud 1.0.5-1
xorgproto 2018.4-1
```

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

[bugzilla-daemon]

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

--- Comment #11 from Hal Gentz  ---
Created attachment 143765
  --> https://bugs.freedesktop.org/attachment.cgi?id=143765=edit
Patch for printing out backtraces for possibly relevant functions.

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

[bugzilla-daemon]

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

[bugzilla-daemon]

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

--- Comment #7 from Samuel Sieb  ---
Of course, as soon as I write that and try again, it crashes every time even
limited to one cpu.  I must have just been lucky that one time or something.

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

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

--- Comment #6 from Samuel Sieb  ---
I can verify that limiting the program to one core allows it to start up.

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

[bugzilla-daemon]

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

Samuel Sieb  changed:

   What|Removed |Added

 CC||sam...@sieb.net

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

[bugzilla-daemon]

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

Tapani Pälli  changed:

   What|Removed |Added

 CC||lem...@gmail.com

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

[bugzilla-daemon]

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

Nicholas Miell  changed:

   What|Removed |Added

 CC||synapt...@gmail.com

--- Comment #5 from Nicholas Miell  ---
*** Bug 105004 has been marked as a duplicate of this bug. ***

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

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

--- Comment #4 from Ray Strode [halfline]  ---
so looking a few stack frames up I see this:

   err = xcb_request_check(c, cookie);•  
   if (err != NULL) {• 
  gc->vtable->destroy(gc);•
  gc = NULL;•  
•   
  __glXSendErrorForXcb(dpy, err);•
  free(err);•  
   }•   

and then __glXSendErrorForXcb does this:

void•
__glXSendErrorForXcb(Display * dpy, const xcb_generic_error_t *err)•
{•
   xError error;•
…
   error.sequenceNumber = err->sequence;•
…
   _XError(dpy, );•
…
}•

So we see the sequence number comes from before a destroy hook which calls into
the driver.  I guess it's possible (maybe?) that the hook is generating
protocol that updates last_request_read.

You could try reordering the gc->vtable->destroy(gc); after the
glXSendErrorForXcb and see if that fixes it. 

If that doesn't work you could set a hardware watchpoint on (((struct
_XDisplay*)dpy)->last_request_read that just prints a backtrace and continues,
so you could see what's updating the last_request_read out from under it.  If
that changes timing too much, you could put backtrace() into arrays in
X_DPY_SET_LAST_REQUEST_READ() and backtrace_symbols from gdb when it tanks.

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

[bugzilla-daemon]

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

--- Comment #3 from Ray Strode [halfline]  ---
In your case the last request accounted for is 84 and the narrowed error event
sequence number is 82.

The code notices 82 is less than 84 and assumes the upper bits were off by one,
but what's really happened (I guess) is somehow the last request accounted for
is getting updated early. maybe events are getting handled out of order

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

[bugzilla-daemon]

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

--- Comment #2 from Ray Strode [halfline]  ---
(In reply to Ray Strode [halfline] from comment #1) 
> I guess if it's gotten to the assertion things have already gone pair shaped
> because that's happening when trying to send the error.
So this isn't right I guess.  The game is doing a series of
glXCreateContextAttribsARB () calls to figure out what GL level / features are
available.  Failure is expected, but blown assertions aren't.

widen() is this:

static void widen(uint64_t *wide, unsigned int narrow)•  
{•   
→   uint64_t new = (*wide & ~((uint64_t)0xUL)) | narrow;•
→   *wide = new + (((uint64_t)(new < *wide)) << 32);•
}•   

which is rather confusing but after staring at it for a bit i'll try to
deconstruct it:

The first parameter, wide, is an in-out variable.  It takes a 64-bit sequence
number as a template, gotten from X_DPY_GET_LAST_REQUEST_READ() and then
returns the output of the function back in the same variable.  The second
parameter, narrow, is a 32-bit sequence number.  X events only have room for
32-bit sequence numbers associated with them at the xlib level, so narrow is
that 32-bit input.  The goal is to recover the lost upper bits, treat that
32-bit sequence number as an offset into a 64-bit number.  We need to find the
upper base bits.  Those, are gleaned from the template passed in.

So the first line of the function extracts the upper 32-bits from the template
and applies them to the 32-bit number.  Now the 32-bit number has been given
its 64 bits back.  But there is a small possibility the 64-bit template and the
event don't actually share the same 32-bits.  new should always be greater than
the template when widened, because the template is X_DPY_GET_LAST_REQUEST_READ
() which is the last request we knew the X server read.  The last request we
knew the X server read should be smaller, because this new event is going to be
the new last request read; we're in a function that updates the last request
read counter known sequence number, and it's happening later in the function. 
If the last request previously accounted for is ending up bigger than this new
event then it means the upper 32-bits of the sequence number have after
actually incremented since then, and the template upper 32-bits are off by one.
So the second line is compensating for that.

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

[bugzilla-daemon]

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

Ray Strode [halfline]  changed:

   What|Removed |Added

 CC||rstr...@redhat.com

--- Comment #1 from Ray Strode [halfline]  ---
can you post an strace -f of the problem happening?

I guess if it's gotten to the assertion things have already gone pair shaped
because that's happening when trying to send the error.

Does the problem go away if you boot with maxcpus=1 ?

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

[bugzilla-daemon]

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@lists.freedesktop.org
  Reporter: nmi...@gmail.com
QA Contact: mesa-dev@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@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:58
#1  0x761b051a in __GI_abort () at abort.c:89
#2  0x761a6da7 in __assert_fail_base (fmt=,
assertion=assertion@entry=0x772b0778 "!xcb_xlib_threads_sequence_lost",
file=file@entry=0x772b05eb "xcb_io.c", 
line=line@entry=259, function=function@entry=0x772b0a28
<__PRETTY_FUNCTION__.14787> "poll_for_event") at assert.c:92
#3  0x761a6e52 in __GI___assert_fail
(assertion=assertion@entry=0x772b0778 "!xcb_xlib_threads_sequence_lost",
file=file@entry=0x772b05eb "xcb_io.c", line=line@entry=259, 
function=function@entry=0x772b0a28 <__PRETTY_FUNCTION__.14787>
"poll_for_event") at assert.c:101
#4  0x7723e32a in poll_for_event (dpy=dpy@entry=0x21ca160) at
xcb_io.c:256
#5  0x7723e3db in poll_for_response (dpy=dpy@entry=0x21ca160) at
xcb_io.c:274
#6  0x7723e6cd in _XEventsQueued (dpy=0x21ca160, mode=)
at xcb_io.c:349
#7  0x77241515 in _XGetRequest (dpy=0x21ca160, type=,
len=4) at XlibInt.c:1707
#8  0x7724162f in _XSeqSyncFunction (dpy=0x21ca160) at XlibInt.c:202
#9  0x77240df3 in _XError (dpy=dpy@entry=0x21ca160,
rep=rep@entry=0x7fffd3e0) at XlibInt.c:1436
#10 0x775591a2 in __glXSendErrorForXcb (dpy=dpy@entry=0x21ca160,
err=err@entry=0x304bd30) at glx_error.c:81
#11 0x77555284 in glXCreateContextAttribsARB (dpy=0x21ca160,
config=0x22b3a20, share_context=0x220c3c0, direct=,
attrib_list=0x7fffd4a0) at create_context.c:119
#12 0x00eac13f in ?? ()
#13 0x00eace22 in ?? ()
#14 0x004654a4 in ?? ()
#15 0x76199401 in __libc_start_main (main=0x4641e0, argc=1,
argv=0x7fffdde8, init=, fini=,
rtld_fini=, stack_end=0x7fffddd8)
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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