[Xcb] xcb_wait_for_reply deadlocking with xcb_property_changed call

Jamey Sharp jamey at minilop.net
Sat Mar 27 12:26:45 PDT 2010

On Sat, Mar 27, 2010 at 4:30 AM, Nicholas Allen
<nick.allen at onlinehome.de> wrote:
> Unfortunately, I'm also experiencing these deadlocks when I call
> xcb_wait_for_event even though I've called xcb_flush just before. I
> thought it was only happening when I used the property util functions
> but I now caught it under the debugger outside of this function. This
> doesn't happen as often as with the property change helper library but
> it still happens occasionally.
> So it seems there is a major thread deadlocking bug in XCB. I don't
> really know how to debug it though. Any suggestions? I'm not at all
> familiar with the XCB code base so any pointers greatly appreciated...

Can you get a wire protocol trace from a session that hangs? With
xtrace, Wireshark, whatever.

Also, can you "thread apply all bt full" in gdb once it hangs and send
us that output? It'll help a lot if you have debugging symbols
available for libxcb; on Debian systems, those are in the libxcb1-dbg

As always, the most helpful thing would be a minimal C test case
demonstrating the bug, but hopefully the protocol trace and the stack
traces will be enough to start forming hypotheses about what's wrong.


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