[Xcb] libxcb/libX11 deadlocking and asserting in multi-threaded use

Nicholas Allen nick.allen at onlinehome.de
Fri Mar 19 01:13:56 PDT 2010

The C program to reproduce this bug is very small and I have attached it.

I compiled it on Linux with the command:

gcc xlib-bug.c -lX11 -lpthread -oxlib-bug

When run you see that it tries to get the next event and never gets one
(ie it deadlocks inside XNextEvent). Sometimes it works for a short
period of time but mostly it deadlocks for me. Sometimes it deadlocks
even before the window is opened.

Is there anything I'm doing wrong in that example code?

Thanks in advance,

Nicholas Allen

Nicholas Allen wrote:
> Hi,
> We have a multi-threaded app that often deadlocks inside libx11 code
> (XNextEvent never returns). We've initialized threading support by
> calling XInitThreads as the very first thing we do. We have no other
> mutexes in use in our code so it seems that the error is within
> libX11/xcb itself and not our own code.
> When the app doesn't lock (very rarely) then after a short time we get
> this assertion:
> ../../src/xcb_io.c:242: process_responses: Assertion `(((long)
> (dpy->last_request_read) - (long) (dpy->request)) <= 0)' failed.
> Aborted
> and the application aborts. We are using Ubuntu Karmic on 64-bit
> machine. In Jaunty we never experienced any problems but since the
> upgrade the problems are very apparent. This further convinces me that
> the problem is more likely in libX11/XCB than our own code. Has anyone
> else noticed these deadlock issues? Are there any workarounds? Were
> there any bug fixes that could have addressed issues like this?
> I will be happy to provide more information if needed and to help with
> debugging and fixing xlib if needed.
> Many thanks in advance,
> Nicholas Allen
> _______________________________________________
> Xcb mailing list
> Xcb at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/xcb

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