X display locking

[Torsten Jager]

Hello!

> What version of XCB are you using?  There were a significant number of 
> thread-related problems introduced when libX11 first switched to using 
> XCB as a backend.  I'd suggest using a libX11 built without XCB, but 
> doing that has gotten a lot harder on recent distributions.

1.7

> This deadlock kind of sounds like this one:
> http://cgit.freedesktop.org/xcb/libxcb/commit/?id=23911a707b8845bff52cd7853fc5d59fb0823cef

Thanks. Contained in 1.9 I use now.
But this was not enough. I also needed a patch from recent libX11.
Seems those lockups have gone with those.

> I think Allen must have been thinking of the Xlib internal LockDisplay 
> and UnlockDisplay functions.  However, those functions aren't necessary 
> unless you need multiple requests from one thread to be atomic w.r.t. 
> other threads using the same display connection.  For your use case, it 
> sounds like you don't need them.

Well, with OpenGL omitting X(Un)LockDisplay () led to significantly
worse performance - kernel mode processor load up to 20%, which used
to be <4% before. Lots of additional context switches and cache
flushes.

Looks like my previous quick fix of bracketing the whole frame
conversion/output sequence is not that bad.

Torsten