http://bugs.freedesktop.org/show_bug.cgi?id=26820
Summary: Sharing contexts crashes on Windows
Product: Mesa
Version: 7.6
Platform: All
OS/Version: Windows (All)
Status: NEW
Severity: critical
Priority: high
Component: Mesa core
AssignedTo: mesa3d-dev@lists.sourceforge.net
ReportedBy: s.zer...@gom.com
Its quite simple to reproduce on Windows using the GDI driver (did not test the
gallium driver but it should suffer fom the same problem):
initialization:
HGLRC rc_to_share = wglCreateContext( hDC );
HGLRC rc_2 = wglCreateContext( hDC );
wglShareLists(rc_to_share, rc_2);
rendering:
glBegin( GL_TRIANGLES );
=> CRASH
The reason seems to be a bug in the _mesa_share_state implementation. This
points the rc_2 context's "Shared" attribut to the "Shared" instance of
rc_to_share. It then realizes that the old "Shared" instance of rc_2 has a
refCount of 0 and deletes the associated memory. Unfortunately the GLcontext
rc_2 has some attributes that still point to the just freed memory.
When calling glBegin the application "crashes" due to an assert in
vbo_exec_vtx_map. There you will find that vbo_exec_context::vtx::bufferobj
still points to GLcontext::Shared->NullBufferObj BUT not of the rc_to_share
context but of the rc_2 context. In other words the bufferobj in the
vbo_exec_context does still point to the memory location that has been freed.
It should point to the memory location of the rc_to_share's
Shared::NullBufferObj.
I cannot tell if other attributes of those structures are concerned but
obviously the _mesa_share_state function does only take care of the
gl_shared_state contents and it does NOT adjust other structure's field that
point to the same addresses as the (then freed) pointers in gl_shared_state. So
there might be more problems around the corner which I have not investigated.
As a small workaround to share contexts on Windows I have added my own function
to the GDI driver. The _mesa_initialize_context function (internally used by
wglCreateContext via WMesaCreateContext) already has a parameter share_lists so
I wrote a function myWglCreateSharingContext (HDC, HDLRC) to feed this
parameter. Apart from the its implementation is the same as
wglCreateContext(HDC).
This workaround works without any problems which leads to the assumption that
only the sharing between two existing contexts is concerned. Creating a new
context as a sharing context right from the start (as you usually do on X11)
does not suffer from problems.
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