Btw, this problem concerns not only OpenGL canvas implementation.
Even with bitblt, some operations are not thread-safe.
And in general, what mechanisms you planning to add to SecureSqueak to
guarantee that some code will get exclusive access to functions of
some device?

For instance, try:
10 timesRepeat: [
   [ Smalltalk logChange: 'say goodbye to' , 1 seconds asDelay wait
asString, ' your .changes file' ] fork.
]

-- 
Best regards,
Igor Stasenko AKA sig.