[Visualpython-users] fix/hack for hanging program

[Fred Y. <fr...@on...>]

As I've mentioned before, I've got a VPython/Tk program that hangs on my
machine, a dual Pentium3 800 system running WinNT4 SP6.  Others have
tried to recreate my problem, but none have seen it.

I've found that with one small change to the C++ code for VPython
itself, I can prevent the deadlock without seeming to cause any other
problems.  Here's the patch:

--- displaylist.cpp.orig        Sat Mar 24 07:09:58 2001
+++ displaylist.cpp     Sat Mar 24 07:12:11 2001
@@ -143,7 +143,7 @@

 void DisplayObject::remove() {
   if (prev) {
-    DisplayObject::read_lock P(prev->mtx);
+    //DisplayObject::read_lock P(prev->mtx);
     prev->next = next;
   }
   if (next) {

--

I've simply disabled the first 'read_lock' in the
DisplayObject::remove() method.  I came to this because my application
always seems to hang at the point where it sets the 'visible' attribute
of an object to false, and this function is called in response to that
action.

So, is there something wrong with the locking scheme in cvisual, or am I
just hacking around a deeper problem such as some glitch in display
drivers?  I admit that I know nothing about OpenGL, much less about what
locking strategy is necessary.  I don't even understand how there can be
contention in my application since all work would seem to be done in the
Tk thread in response to callback functions associated with the Slider
objects.  But here too, I don't understand just what threads are
involved and how they can interleave.

It does bother me that the insert() and remove() methods of
DisplayObject both create two locks, not releasing the first one before
acquiring the second even though the first seems no longer to be needed
when the second is acquired.  I tried hacking the code to explicitly
delete the first lock as soon as possible, but that version still
deadlocks.

My problematic application is still at
<http://www.ontosys.com/src/residue.py>.

Here's the output from glinfo.py, showing my private version of
cvisual.dll.  My video card is a 3Dfx Voodoo-3 3000 16MB AGP, set at
1600 x 1200 x 32 at 85 Hz:

--
$ python glinfo.py
Visual-2001-02-27
  e:\programs\python20\dlls\cvisual.dll Sat Mar 24 07:12:18 2001
  e:\programs\python20\visual\__init__.py Tue Feb 27 21:58:28 2001
  e:\programs\python20\visual\graph.py Tue Feb 20 20:35:42 2001
  d:\winnt\system32\opengl32.dll Thu Nov 18 10:04:00 1999
OpenGL renderer active.
  Vendor: Microsoft Corporation
  Version: 1.1.0
  Renderer: GDI Generic
  Extensions:
                       GL_WIN_swap_hint
                            GL_EXT_bgra
                GL_EXT_paletted_texture

-- 
Fred Yankowski           fr...@On...      tel: +1.630.879.1312
Principal Consultant     www.OntoSys.com       fax: +1.630.879.1370
OntoSys, Inc             38W242 Deerpath Rd, Batavia, IL 60510, USA