First of all, does the app call XInitThreads before any other libX11 functions are called (on any thread)?

If so, any chance you can try libxcb 1.11.1 or a current Git snapshot?

Yes, XInitThreads() is called at startuo (main function) before any other libX11 functions are called from any threads of the app.

Problem still exists with libxcb 1.11.1 (I am buiding and installing it manually though. Does newer Xlib also need to be built, or is xcb sufficient?)

(In reply to Vasilis LIaskovitis from comment #0)
[...]
> I see a hang in xcb_wait_for_reply() for request #31161:
>  
> Possibly relevant part of xtrace:
> 000:>:4d8a:32: Reply to SwapBuffers: swap_hi=0 swap_lo=31161
> 000:>:4d8b:32: Reply to GetInputFocus: revert-to=Parent(0x02)
> focus=0x02e000e0
> 000:<:24d8c: 20: DRI2-Request(155,7): GetBuffersWithFormat
> drawable=0x04400006 attachments={attachment=BackLeft(0x00000001)
> format=0x00000020};
> 000:>:24d8c: Event DRI2-BufferSwapComplete(102) drawable=0x00000002
> ust_hi=71303174 ust_lo=0 msc_hi=1858207713 msc_lo=0 sbc_hi=109393
> sbc_lo=31161

Why is this part relevant? It's 0x31161-0x24d8c = 50133 requests before the hang, so apparently things still work for 50k requests. That should be a long time.

> And:
> 000:<:31161:  4: Request(43): GetInputFocus  
> 000:>:31161: Event DRI2-InvalidateBuffers(103) drawable=0x04400006

So if I understand this correctly, you are saying that requests #31161 is a GetInputFocus request and xtrace never saw a reply to it, right?
Isn't the bug then in the server for not replying to GetInputFocus?

> Full backtrace at: http://pastebin.com/rQCJm64C

Thread 16 is in radeon_something, waits on a condition
Thread 15 waits for the vaapi lock(?) (pthread_mutex_lock() from within gst_vaapisink_x11_handle_events())
Thread 14 waits on a condition inside gst_data_queue_push() (hm?!? Is some queue full, or why does it wait?)
Thread 13 waits on a condition inside gst_task_func (dunno what this is)
Thread 12 waits on a condition inside gst_vaapidecode_handle_frame() (and has a deep backtrace with lots of gst stuff)
Thread 11 waits in gst_queue_chain_buffer_or_list() (deep backtrace)
Thread 10 waits on a condition inside gst_queue_loop()
Thread 9 waits on a condition inside gst_base_sink_wait_preroll() (deep backtrace)
Thread 8 poll()s for a reply to request 31161 (called from vl_dri2_get_flush_reply()) (deep backtrace)
Thread 7 waits on a condition inside ??() in libavcodec.so
So do thread 6, 5, 4, 3.
Thread 2 poll()s something pulseaudio related (pa_mainloop_poll())
Thread 1 *also* poll()s the xcb connection (wait_for_reply() on request 212481, called from _XReply(), called from DRI2GetBuffersWithFormat, called eventually from st_Clear() (something from mesa), called from main())

Ohhh... thread 1 and thread 8 wait on different XCB connections. I have no clue about the internals of mesa etc, but could it be that something did a GrabServer and that's why the GetInputFocus (and DRI2GetBuffersWithFormat) isn't replied to? Since each thread involving libxcb has its own connection, I really don't think that the deadlock is inside of libxcb.

@Vasilis: How do you know that this is a deadlock? Does your X11 server still work correctly otherwise? I'd expect both of these pending requests to get replies.

Also, could you do something like this ('thread 1' would switch to, well, thread 1 and frame 2 to the frame with `_xcb_conn_wait()` so that we get a reference to the XCB connection, the various prints print the state of the XCB connection):
thread 1
frame 2
print c
print c.in->current_reply
print c.in->events
print c.in->pending_replies

(In reply to Uli Schlachter from comment #3)
> (In reply to Vasilis LIaskovitis from comment #0)
> [...]
> > I see a hang in xcb_wait_for_reply() for request #31161:
> >  
> > Possibly relevant part of xtrace:
> > 000:>:4d8a:32: Reply to SwapBuffers: swap_hi=0 swap_lo=31161
> > 000:>:4d8b:32: Reply to GetInputFocus: revert-to=Parent(0x02)
> > focus=0x02e000e0
> > 000:<:24d8c: 20: DRI2-Request(155,7): GetBuffersWithFormat
> > drawable=0x04400006 attachments={attachment=BackLeft(0x00000001)
> > format=0x00000020};
> > 000:>:24d8c: Event DRI2-BufferSwapComplete(102) drawable=0x00000002
> > ust_hi=71303174 ust_lo=0 msc_hi=1858207713 msc_lo=0 sbc_hi=109393
> > sbc_lo=31161
> 
> Why is this part relevant? It's 0x31161-0x24d8c = 50133 requests before the
> hang, so apparently things still work for 50k requests. That should be a
> long time.
> 

You are right, this looks irrelevant.. not very familiar xith X protocol, I was thinking sbc_lo could refer to a request ID.

> > And:
> > 000:<:31161:  4: Request(43): GetInputFocus  
> > 000:>:31161: Event DRI2-InvalidateBuffers(103) drawable=0x04400006
> 
> So if I understand this correctly, you are saying that requests #31161 is a
> GetInputFocus request and xtrace never saw a reply to it, right?
> Isn't the bug then in the server for not replying to GetInputFocus?
> 
> > Full backtrace at: http://pastebin.com/rQCJm64C
> 
> Thread 16 is in radeon_something, waits on a condition
> Thread 15 waits for the vaapi lock(?) (pthread_mutex_lock() from within
> gst_vaapisink_x11_handle_events())
> Thread 14 waits on a condition inside gst_data_queue_push() (hm?!? Is some
> queue full, or why does it wait?)
> Thread 13 waits on a condition inside gst_task_func (dunno what this is)
> Thread 12 waits on a condition inside gst_vaapidecode_handle_frame() (and
> has a deep backtrace with lots of gst stuff)
> Thread 11 waits in gst_queue_chain_buffer_or_list() (deep backtrace)
> Thread 10 waits on a condition inside gst_queue_loop()
> Thread 9 waits on a condition inside gst_base_sink_wait_preroll() (deep
> backtrace)
> Thread 8 poll()s for a reply to request 31161 (called from
> vl_dri2_get_flush_reply()) (deep backtrace)
> Thread 7 waits on a condition inside ??() in libavcodec.so
> So do thread 6, 5, 4, 3.
> Thread 2 poll()s something pulseaudio related (pa_mainloop_poll())
> Thread 1 *also* poll()s the xcb connection (wait_for_reply() on request
> 212481, called from _XReply(), called from DRI2GetBuffersWithFormat, called
> eventually from st_Clear() (something from mesa), called from main())
> 
> Ohhh... thread 1 and thread 8 wait on different XCB connections. I have no
> clue about the internals of mesa etc, but could it be that something did a
> GrabServer and that's why the GetInputFocus (and DRI2GetBuffersWithFormat)
> isn't replied to? Since each thread involving libxcb has its own connection,
> I really don't think that the deadlock is inside of libxcb.
> 
> @Vasilis: How do you know that this is a deadlock? Does your X11 server
> still work correctly otherwise? I'd expect both of these pending requests to
> get replies.
> 
> Also, could you do something like this ('thread 1' would switch to, well,
> thread 1 and frame 2 to the frame with `_xcb_conn_wait()` so that we get a
> reference to the XCB connection, the various prints print the state of the
> XCB connection):
> thread 1
> frame 2
> print c
> print c.in->current_reply
> print c.in->events
> print c.in->pending_replies

pending_replies is 0

(gdb) print c.in->current_reply
$4 = (struct reply_list *) 0x0
(gdb) print c.in->events
$5 = (struct event_list *) 0x7f0b1c008730
(gdb) print c.in->pending_replies
$6 = (struct pending_reply *) 0x0

Looking at all threads in bt, this may be a completely different issue with gstreamer queues.. we can maybe close the issue and reopen if I find something that really proves it is an xcb/xserver issue (or if you have another idea)

Just an explanation for that debug info:
There are no replies that XCB already received, but did not return to any caller. Some events were received, but not yet returned to the application (nothing called xcb_poll/wait_for_event()). The empty "pending_replies" list means that no answer to checked requests is pending (hm? Is this the thread where libX11 owns the socket?).

Besides that, I don't know. I'd blame the X server for not answering and the non-X11 threads don't tell me anything.

Seems like i'm hit by the same bug. In my case, it hangs in OpenGL games (glxgears seems to be unaffected) randomly when i switch workspaces. With StarRuler2 i only have one thread hanging while communicating with X, and here's the backtrace:

		#0  0x00007ffff5d05db3 in poll () from /lib64/libc.so.6
		#1  0x00007ffff55769e2 in _xcb_conn_wait () from /usr/lib64/libxcb.so.1
		#2  0x00007ffff55789b7 in wait_for_reply () from /usr/lib64/libxcb.so.1
		#3  0x00007ffff5578b71 in xcb_wait_for_reply64 () from /usr/lib64/libxcb.so.1
		#4  0x00007ffff7a3e288 in _XReply () from /usr/lib64/libX11.so.6
		#5  0x00007ffff7f891c7 in DRI2GetBuffersWithFormat (dpy=0x555555a6c410, drawable=163577862, width=width@entry=0x555555e20a28, height=height@entry=0x555555e20a2c, attachments=0x7fffffffd620, count=1, outCount=0x7fffffffd69c)
		    at ../mesa-18.2.0/src/glx/dri2.c:485
		#6  0x00007ffff7f89596 in dri2GetBuffersWithFormat (driDrawable=<optimized out>, width=0x555555e20a28, height=0x555555e20a2c, attachments=<optimized out>, count=<optimized out>, out_count=0x7fffffffd69c, loaderPrivate=0x555555e20c80)
		    at ../mesa-18.2.0/src/glx/dri2_glx.c:900
		#7  0x00007fffeb3b57ef in intel_query_dri2_buffers (brw=brw@entry=0x555555c6c1b0, drawable=drawable@entry=0x555555e20a00, buffers=buffers@entry=0x7fffffffd6a0, buffer_count=buffer_count@entry=0x7fffffffd69c)
		    at ../mesa-18.2.0/src/mesa/drivers/dri/i965/brw_context.c:1531
		#8  0x00007fffeb3b592f in intel_update_dri2_buffers (brw=brw@entry=0x555555c6c1b0, drawable=drawable@entry=0x555555e20a00) at ../mesa-18.2.0/src/mesa/drivers/dri/i965/brw_context.c:1370
		#9  0x00007fffeb3b5b3d in intel_update_renderbuffers (context=context@entry=0x555555c660a0, drawable=drawable@entry=0x555555e20a00) at ../mesa-18.2.0/src/mesa/drivers/dri/i965/brw_context.c:1426
		#10 0x00007fffeb3b5baf in intel_prepare_render (brw=brw@entry=0x555555c6c1b0) at ../mesa-18.2.0/src/mesa/drivers/dri/i965/brw_context.c:1445
		#11 0x00007fffeb3b079e in brw_clear (ctx=0x555555c6c1b0, mask=272) at ../mesa-18.2.0/src/mesa/drivers/dri/i965/brw_clear.c:254
		#12 0x00007fffeb5b1fe7 in clear (no_error=false, mask=16640, ctx=0x555555c6c1b0) at ../mesa-18.2.0/src/mesa/main/clear.c:221
		#13 _mesa_Clear (mask=<optimized out>) at ../mesa-18.2.0/src/mesa/main/clear.c:242
		#14 0x00005555557d77c9 in render::GLDriver::setRenderTarget(render::Texture*, bool) ()
		#15 0x000055555582ecdb in renderFrame(scripts::Manager*, scripts::Manager*) ()
		#16 0x0000555555813eed in tickMenu() ()
		#17 0x00005555555a842c in main ()

Can i do anything about it?

Seems like it is fixed if i build xf86-video-intel with --with-default-dri=3

This sounds like the issue is that DRI2 is broken in the X server itself, which I believe was fixed last year.